chore: add Copilot custom instructions for code review

.github/copilot-instructions.md

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+# Copilot Instructions for mcpls
+
+mcpls is a bridge between MCP (Model Context Protocol) and LSP (Language Server Protocol).
+AI clients speak MCP to mcpls; mcpls spawns and communicates with language servers over LSP.
+
+```
+AI Client ←→ [MCP/stdio] ←→ mcpls ←→ [LSP/stdio] ←→ language server
+```
+
+Key crates: `mcpls-core/src/bridge/`, `lsp/`, `mcp/`, `config/`.
+
+## Type safety
+
+Prefer newtypes over primitive aliases for domain values. A raw `u32` for a line number
+and a raw `u32` for a column are indistinguishable to the compiler; a `Line(u32)` and
+`Column(u32)` make transpositions a compile error.
+
+Encode protocol state in the type system where possible. A document that has been opened
+and one that has not should ideally be different types, not the same type with a boolean
+field. Typestate prevents calling operations that are only valid on open documents.
+
+Avoid stringly-typed dispatch. Matching on `&str` method names to branch protocol
+behaviour should be replaced with a typed enum as soon as the set of methods is known
+and bounded.
+
+`Option<T>` and `Result<T, E>` must not be unwrapped with `.unwrap()` or `.expect()` in
+production code paths. Use `?`, `if let`, `let-else`, or explicit error mapping.
+
+## Idiomatic Rust
+
+Prefer iterator adapters over manual loops. `map`, `filter`, `flat_map`, `collect`, and
+`fold` express intent more clearly and are easier to compose than `for` with a mutable
+accumulator.
+
+Avoid unnecessary heap allocation. Prefer `&str` over `String` in function signatures
+where ownership is not required. Prefer `Cow<str>` when a function sometimes borrows and
+sometimes owns.
+
+Use `let-else` (stabilised in Rust 1.65) to reduce nesting when early return on `None`
+or `Err` is needed:
+```rust
+// prefer
+let Some(value) = option else { return Err(...) };
+// over
+let value = match option { Some(v) => v, None => return Err(...) };
+```
+
+Derive `Clone`, `Debug`, `PartialEq` on data types unless there is a specific reason not
+to. Their absence makes testing harder and the omission is rarely intentional.
+
+## Architecture
+
+A component that bridges two protocols (MCP and LSP) must not let either protocol's
+failure mode affect the other. LSP request timeouts must not stall MCP response
+delivery, and LSP push notifications must not block MCP request handling.
+
+Shared mutable state must be scoped as narrowly as possible. A `Mutex` that guards a
+large composite struct is a concurrency bottleneck; prefer separate locks for
+independent sub-components (e.g. a cache and a client are independent).
+
+A `Mutex` guard must never be held across an `.await` point that involves I/O. Doing so
+holds the lock for the full duration of the network round-trip, blocking every other
+task that needs the lock. Extract the guarded value before the `.await` or restructure
+so the lock is released first.
+
+## Async
+
+Prefer `tokio::join!` or `tokio::try_join!` over sequential `.await` chains when
+operations are independent. Sequential awaits serialise work that could run concurrently.
+
+Blocking operations (`std::fs`, `std::thread::sleep`, CPU-heavy loops) must not run on
+the async executor. Use `tokio::fs`, `tokio::time::sleep`, or `tokio::task::spawn_blocking`
+for work that would block a task for more than a few microseconds.
+
+Use `tokio::sync::mpsc` for async-to-async channels. Use `std::sync::mpsc::sync_channel`
+only for bridging a synchronous context to async, and always with `try_send` when the
+intent is to drop events under backpressure — `send` blocks on a full channel.
+
+Prefer `tokio::select!` with a cancellation token over bare `loop { recv().await }`
+for background tasks that must shut down cleanly.
+
+## API currency and MSRV
+
+When reviewing code, check whether newer stable Rust APIs would simplify it. If a
+simpler or safer API was stabilised after the current `rust-version` in `Cargo.toml`,
+suggest bumping MSRV and using the new API. Document the bump in CHANGELOG under
+`### Changed`.
+
+Examples of APIs worth adopting when MSRV permits:
+- `let-else` expressions (1.65) — replace verbose `match`/`unwrap_or_else` for early return
+- `std::io::read_to_string` on a `File` handle (1.0, but pairing with `.metadata()` on the same
+  handle closes TOCTOU windows — prefer over separate `stat` + `open`)
+- `OnceLock` (1.70) — prefer over `lazy_static` or `once_cell` for static initialisation
+- `is_some_and` / `is_none_or` (1.70) — replace `map(|v| ...).unwrap_or(false)` patterns
+- `Iterator::array_chunks` (nightly → stable tracking) — watch stabilisation status
+
+When a dependency provides a feature already in `std`, prefer `std`. Fewer dependencies
+reduce supply-chain risk and compile time.

.github/instructions/changelog.instructions.md

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+---
+applyTo: "CHANGELOG.md"
+---
+
+## Changelog format
+
+Breaking changes to public APIs must appear under `### Changed` with an explicit
+"Breaking change:" prefix. A `### Fixed` entry alone is not sufficient — downstream
+users scanning the changelog for breakage will miss it.
+
+Additions of `#[non_exhaustive]` to existing public enums or structs are breaking
+changes for crates that match exhaustively. Document the required migration (add a
+wildcard arm, update struct initialisation) in the entry.
+
+Every entry must reference the PR or issue number: `(#NNN)`.
+
+New entries go under `## [Unreleased]`. Do not add entries directly under a versioned
+section — version assignment happens in the release PR.

.github/instructions/ci.instructions.md

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+---
+applyTo: ".github/workflows/**,deny.toml,.cargo/**"
+---
+
+## Feature flag parity
+
+Feature flag sets in CI jobs must match the pre-commit commands in `CLAUDE.md` exactly.
+Divergence causes "passes locally, fails CI" failures where neither side is obviously
+wrong.
+
+The rustdoc job must set `RUSTDOCFLAGS="-D warnings"`. Without it, broken intra-doc
+links and missing `///` on public items pass silently in CI.
+
+Nightly toolchain is required only for `cargo fmt`. All other jobs must use stable to
+avoid failures caused by nightly regressions unrelated to this project.
+
+## Dependency and license policy
+
+New dependencies require a license check in the same PR. `cargo deny check licenses`
+fails fast and blocks the pipeline — do not merge before it passes.
+
+`cargo deny check advisories` must be clean. Suppress an advisory only with an explicit
+`ignore` entry and a comment explaining why the vulnerable code path is unreachable.
+
+When a newer stable `std` API covers functionality provided by a dependency, suggest
+removing the dependency and bumping `rust-version` instead. Fewer dependencies reduce
+compile time and supply-chain risk. Document the MSRV bump in CHANGELOG.
+
+## MSRV tracking
+
+`rust-version` in `Cargo.toml` is the enforced minimum. The CI matrix must include a
+job that builds and tests on exactly that version to catch accidental use of newer APIs.
+Without such a job, MSRV guarantees are not verifiable.

.github/instructions/mcp-tools.instructions.md

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+---
+applyTo: "crates/mcpls-core/src/mcp/**"
+---
+
+## Type safety
+
+Tool parameter structs must use domain types, not raw primitives, for values with
+protocol-specific conventions. Position fields (line, column) must be documented as
+1-based in both the `///` doc comment and the `#[schemars(description)]` — the MCP↔LSP
+coordinate conversion happens in the bridge layer, not here.
+
+Path parameters must accept absolute paths. State this in the schema description; AI
+clients use it to form requests correctly.
+
+New tool structs require `#[derive(JsonSchema)]`. Omitting it silently removes the tool
+from the schema exposed to clients — they cannot discover or invoke it.
+
+## Error handling
+
+Tool handlers must return structured MCP error responses on failure, never panic.
+`unwrap()` and `expect()` in handler code kill the entire server process and drop all
+active client sessions. Use `?` with proper error mapping instead.
+
+All error variants must carry enough context for the caller to understand what went
+wrong. A bare `"internal error"` string is not actionable. Include the file path,
+method name, or other relevant detail.
+
+## Idiomatic dispatch
+
+When adding a new tool, register it in both the capability advertisement (tool list)
+and the dispatch match arm. A tool missing from the list is undiscoverable; a tool
+missing from dispatch panics at runtime. A compile-time check (e.g. a const assertion
+or a test that calls every listed tool name through dispatch) is preferable to relying
+on manual review.
+
+Prefer exhaustive match arms over wildcard catch-alls in dispatch. A wildcard silently
+swallows unhandled tool names; exhaustive matching makes unregistered names a compile
+error.

.github/instructions/rust-bridge.instructions.md

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+---
+applyTo: "crates/mcpls-core/src/bridge/**,crates/mcpls-core/src/lsp/**,crates/mcpls-core/src/lib.rs"
+---
+
+## Type safety
+
+Position types (line, column) must be distinct newtypes, not bare `u32`. The MCP
+convention (1-based) and the LSP convention (0-based) must be expressed in the type
+system so that passing an MCP position directly to an LSP call is a compile error.
+
+Protocol message variants must be an enum, not a string-matched dispatch. Matching on
+`"textDocument/hover"` strings is fragile; a closed enum of known methods makes
+unhandled cases visible at compile time and exhaustiveness-checked.
+
+`InboundMessage` and similar enums that may gain variants in future protocol versions
+must be `#[non_exhaustive]` so downstream crates are not broken by additions.
+
+## Document state
+
+Caching document content alongside a filesystem signature (`mtime` + size) prevents
+stale reads after external edits. On every access, stat the file and compare against
+the cached signature; re-read and re-notify the LSP server on mismatch.
+
+Stat and read must use the same file handle to close the TOCTOU window. Open the file
+once with `tokio::fs::File::open`, call `.metadata().await` on the handle, then read
+through the same handle. A separate `tokio::fs::metadata` call before `read_to_string`
+leaves a window where an atomic `rename` can change the file between the two calls.
+
+Document version numbers must be strictly monotone per URI. On overflow, reset to 1
+rather than saturating — after `didClose`/`didOpen` the server treats the document as
+fresh and the version value is irrelevant to continuity.
+
+## Diagnostics pipeline
+
+When a feature draws from multiple independent sources (pull request + push cache),
+fetch them concurrently with `tokio::try_join!` or `tokio::join!`. Awaiting them
+sequentially serialises work that has no data dependency.
+
+A cache miss must be distinguishable from an empty result. Returning `Vec::new()` for
+both "no errors" and "not yet analysed" is ambiguous to callers. Use an `Option<Vec>`
+or a dedicated status type to express the difference.
+
+Deduplication keys should prefer structured fields (error code) over free-form message
+text. When a code is present, key on `(range, severity, code)`. The doc comment must
+describe the actual key — a mismatch between the comment and the implementation causes
+silent behavioural bugs.
+
+## Concurrency and lock scope
+
+Background notification pumps must hold only the narrowest lock needed (e.g. a
+`Mutex<Cache>`, not a `Mutex<WholeTranslator>`). A pump that shares a lock with request
+handlers will stall when a handler holds the lock across a long-running await, causing
+the notification channel to fill and the in-flight response to be undeliverable.
+
+Never hold a `MutexGuard` across an `.await`. Extract needed values before the await,
+or restructure so the guard is dropped before any I/O begins.
+
+## File watcher
+
+Glob patterns from external registrations must be matched against full absolute paths.
+`globset` does not implicitly anchor bare patterns to a directory prefix — a pattern
+like `*.rs` only matches a bare filename. Prepend `**/` to patterns that lack a leading
+`/` or `**/` before compiling them into a `GlobSet`.
+
+Noise filtering (build artifacts, VCS directories) must run before events enter any
+bounded channel, not after. Filtering post-send wastes channel capacity and can stall
+the OS watcher thread when the channel fills during high-churn builds.
+
+Use `try_send` for the handoff from the OS watcher callback to the async processing
+loop. The watcher callback runs on a system thread; blocking it with `send` on a full
+channel stalls the OS-level event delivery.
+
+## LSP client
+
+Server-to-client requests (messages with both `method` and `id`) require a JSON-RPC
+response. Unhandled methods must return error code `-32601` (Method Not Found). Dropping
+them silently causes the LSP server to wait indefinitely for an acknowledgement.
+
+`id`-only messages (no `method`, `result`, or `error`) are protocol errors and must not
+be coerced into a successful `null` response.

.github/instructions/rust-config.instructions.md

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+---
+applyTo: "crates/mcpls-core/src/config/**,deny.toml,Cargo.toml,crates/*/Cargo.toml"
+---
+
+## Type safety and idiomatic config
+
+Config option enums must derive `Default` with `#[default]` on the intended default
+variant rather than implementing `Default` manually. This keeps the default co-located
+with the type definition and is less error-prone.
+
+Boolean config fields that represent a choice between more than two states should be
+`enum`, not `bool`. A `bool` cannot be extended without a breaking change; an enum can
+gain variants under `#[non_exhaustive]`.
+
+New `#[serde(default)]` fields must have a test that deserialises a config with the
+field omitted and asserts the default value. Cover both JSON and TOML — parsers can
+behave differently on edge cases like empty strings and absent vs. null keys.
+
+Enum config options must have a round-trip test for every variant. A typo in
+`#[serde(rename_all = "...")]` silently changes the on-disk format and breaks existing
+user configs without a compile error.
+
+## MSRV and dependencies
+
+When introducing a new dependency, check whether the functionality is already available
+in `std` for the current MSRV. Prefer `std` over external crates to reduce
+compile time and supply-chain surface.
+
+New crates require a license check in the same PR. Add the license to `deny.toml`
+before merging — `cargo deny check licenses` fails fast and blocks the pipeline.
+
+If a new stable Rust API (e.g. `OnceLock` at 1.70 replacing `once_cell::sync::OnceCell`)
+makes an existing dependency redundant, suggest removing the dependency and bumping
+`rust-version` instead. Document the bump in CHANGELOG.

.github/instructions/tests.instructions.md

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+---
+applyTo: "crates/mcpls-core/tests/**,crates/mcpls-core/src/**/tests.rs,crates/mcpls-core/src/**/*_tests.rs"
+---
+
+## Test design
+
+Tests for async code must use `#[tokio::test]` and exercise the actual async paths, not
+synchronous wrappers that bypass scheduler interactions. Concurrency bugs (lock
+contention, channel saturation) are only reachable through genuine async execution.
+
+Tests that require an external binary must be marked `#[ignore = "Requires <binary>
+in PATH"]`. CI runners do not have language server binaries; an unmarked test will fail
+on every clean run.
+
+## Correctness invariants to cover
+
+Any code path that pairs a filesystem operation with a subsequent network notification
+(open, close, re-open after external edit) must have a test that exercises the full
+sequence using a mock network client and a real temporary file — not just the filesystem
+primitive in isolation.
+
+Glob matching tests must use absolute path strings as input. Passing a bare filename
+to a glob set will pass even when the production code fails on absolute paths, because
+`globset` anchoring behaviour differs between the two cases.
+
+Round-trip tests for serialisable types must cover every variant, and must assert the
+serialised string value, not just that deserialisation succeeds. A `rename_all` typo
+produces a wrong on-disk format that round-trips correctly in isolation.
+
+## Idiomatic test code
+
+Prefer `assert_eq!` over `assert!(a == b)` — `assert_eq!` prints both values on
+failure, making diagnosis faster.
+
+Use `tempfile::tempdir()` for tests that need real filesystem paths. Hard-coded `/tmp`
+paths cause test interference when multiple test threads run in parallel.
+
+Prefer `#[tokio::test]` over `tokio::runtime::Runtime::block_on` in test bodies —
+`block_on` does not integrate with tokio's test utilities (`time::pause`,
+`time::advance`) needed for testing debounce and timeout logic.