Add unified Rust agent guidelines

AGENTS.md

@@ -0,0 +1,62 @@
+# Rust Project Guidelines
+
+## General Rust Guidance (common to all projects)
+
+- Prefer idiomatic Rust conventions.
+- Never use `.unwrap()`, `.expect()`, `panic!()`, or any other function, method, or
+  macro that unwinds the stack. Return `Result` with meaningful errors instead.
+  - These **are** allowed in tests.
+- Unless there are naming conflicts or you are building Rust macros, import the
+  object instead of using a qualified path.
+- Only include comments that explain **why** code does something, not how or what
+  it does.
+  - If a comment describes follow-up work, reference its Linear issue number.
+- Avoid the `let _ = ...;` / `let _name = ...;` pattern — just call the function.
+  Allowed only when required for lifetime behavior.
+- Use concrete types that derive `serde::Serialize` / `serde::Deserialize` instead
+  of raw JSON via `serde_json`. Use `serde_json::Value` only in unit tests or at
+  genuinely dynamic, fixture, or interoperability boundaries, converting to typed
+  data as early as practical.
+
+## Dependencies
+
+- Reuse existing workspace crates and utilities before adding a dependency.
+- A crate with more than 100,000 downloads
+  on crates.io may be added without asking; otherwise ask first.
+
+## Post-change checks (all projects)
+
+Run from the project directory after every code edit:
+
+```sh
+cargo +nightly fmt
+cargo clippy -- -D warnings
+```
+
+## Commits and pull requests
+
+- Keep commit messages to at most two sentences.
+- If a Linear ticket is available, start the commit subject (and PR title) with the
+  Linear issue key, e.g. `ENG-4456: Set up existing creed crate for Rust QA`.
+- Include the Linear issue link in the PR body when the PR corresponds to an issue.
+- Squash multiple commits belonging to the same Linear issue before pushing or when
+  updating an open PR (prefer amend/rebase of fixups), unless they are genuinely
+  separate reviewable units.
+
+## Unit tests
+
+- Do not add unit tests that merely mirror literals, field assignment, `Default`,
+  trivial match arms, or the implementation line by line.
+- Add a unit test when it exercises behavior that could plausibly regress without
+  an obvious compile failure: non-trivial algorithms, subtle serialization
+  semantics, date or money calculations, state transitions, authorization, or
+  validation edge cases.
+- Applies to inline `#[cfg(test)]` tests; integration/E2E suites are
+  governed separately.
+
+## Code review
+
+- Library version compatibility: do not request compatibility code, polyfills,
+  fallbacks, or workarounds for older library versions unless repository evidence
+  (manifests, lockfiles, runtime constraints) shows the older version is currently
+  in use or explicitly supported. Review against the resolved version otherwise.

CLAUDE.md

@@ -0,0 +1 @@
+@AGENTS.md