feat: inline edit for local diffs

[imfing]

Summary

Adds an inline edit experience for local (working-tree) diffs, built on the @pierre/diffs 1.4 edit-session API:

• Edit file in the file actions menu starts an edit session on that diff (local mode only; PR/branch diffs review committed states and stay read-only)
• The file header swaps to Cancel / Save while editing; Save accepts the session and writes the completed new-file contents to disk via a new PUT /api/worktree-file endpoint, Cancel rejects and restores the original diff
• The diff recomputes live while typing (header stats, line numbers, word-level highlights); the file watcher picks up the save and reloads the diff

Editable: modified files, renames with changes, new (untracked) files. Not editable: deleted files and pure renames (no hunks, so no rows to edit).

Implementation notes

• git::write_worktree_file mirrors read_worktree_file's path validation (safe relative path, canonicalized under the repo root) and only overwrites existing files; contents capped at the blob hydration limit
• EditProvider + CodeView.onItemEditComplete wire up the 1.4 session lifecycle; edit toggles bump the item version since updateItem ignores same-version records
• Local-mode loadDiffFiles now reads the new side from the worktree: the index-line oid of a worktree diff is computed on the fly and generally absent from the object database (blob hydration 404'd)
• New files: patch-parsed diffs are all isPartial, and Pierre only hydrates change/rename diffs, so a new file's edit session never started. When editing starts, the item gets a copy marked complete (accurate, since a new file's patch holds every line) with a fresh cacheKey, because Pierre treats diffs with equal keys as the same target
• Diff reloads (SSE/poll) are deferred while a session is active so a remount can't tear down the editor mid-edit; deferred reloads fire on session end
• The n/p/m shortcuts are suppressed during an edit: the editor's input lives in shadow DOM, so the document-level handler would otherwise steal those keystrokes

Also fixes: stale diff after a file returns to HEAD

The watcher only broadcast when a touched path appeared in the current git status. A file saved back to its HEAD content drops out of status, so no refresh was sent and the page kept an empty -0 +0 entry. This predates the edit feature (saving back to HEAD from any external editor hit it too). The watcher now keeps the previous status and also broadcasts when a touched path was changed before and is clean now.

Test plan

• pnpm test (oxlint + tsc/vite + cargo test) and cargo clippy -D warnings pass
• New Rust tests: write_worktree_file (overwrite, no-create, traversal rejection) and a watcher regression test that fails without the fix
• Verified end-to-end with headless Chromium against a scratch repo covering each diff shape:
  • Modified file with two hunks: edit + inserted line saved correctly, sidebar totals match git diff HEAD
  • Rename with changes: saves correctly
  • Edit back to HEAD content: file drops out of the diff after save
  • New untracked file: edits and saves
  • Pure rename: no Edit option
  • Cancel: view reverts, disk untouched