refactor(operations): fold FsVaultWriter into the disk provider

[AlexMost]

Refs #114

PR 1 of 3 for the consolidate-vault-writes change. FsVaultProvider becomes the single module that performs every note write over a vault root, with one name ⊕ path rule resolved at one depth and one filesystem-error taxonomy for notes that already exist.

What changes

• One disk module. replaceInNote / replaceFullBody move onto VaultProvider keyed on a NoteIdentifier and are implemented by FsVaultProvider. src/lib/obsidian/vault-writer.ts is deleted, along with IVaultEntry.writer, IVaultEntryDeps.writerFactory, and the writerFactory wiring in src/server.ts. Its unit suite folds into test/operations/fs-vault-provider/edit-note.test.ts, message assertions included.
• One name ⊕ path rule. resolveIdentifier (tool-helpers.ts) is now the only implementation — the inline XORs in create-note.ts and edit-note.ts and the non-XOR guard inside FsVaultProvider.createNote are gone. Every write tool hands an unresolved identifier down; the module resolves it in one of two named modes, resolveExisting (basename index) or resolveNew (.obsidian/app.json's newFileLocation).
• One fs mapping over existing notes. Private readRaw → NOT_FOUND / READ_FAILED and writeRaw → WRITE_FAILED, shared by the edit and property paths, over an injectable readFile / writeFile seam carried over from the deleted writer. createNote deliberately keeps its own NOTE_EXISTS / CREATE_FAILED taxonomy (different flags, and the headless-vault-operations spec mandates it), and readDaily keeps its spec-pinned NOT_FOUND message — both now carry a comment saying so.

set_property and remove_property gain WRITE_FAILED / READ_FAILED coverage they have never had.

Three behaviour changes a reviewer should meet here rather than in the diff

1. create_note with both name and path now reports details.field: "path" (was "name").
2. create_note with neither now says "Provide exactly one of name or path" (was "Provide name or path") — matching what the other three write tools already say.
3. edit_note with replace: '' plus an unresolvable name now fails INVALID_ARGUMENT before touching disk (was NOT_FOUND) — arguments are validated before I/O.

Same error codes in all three cases; no test or doc pinned either string. Everything else about the client-visible contract — tool schemas, parameter names, output shapes, codes and their messages — is unchanged.

Not in this PR

listTags / listProperties still hang off VaultProvider (PR 2 moves them to free functions over a VaultReader and drops computeVaultOverview to three deps). The architecture docs and ADR-0016 land in PR 3, with the archive.

Verification

npm test (109 files / 1355 tests), npm run lint, and npm run typecheck all pass. openspec validate consolidate-vault-writes passes. Acceptance greps from the plan hold: one construction site for the identifier rule, one for WRITE_FAILED, and no VaultWriter / writerFactory reference left in src/ or test/.

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