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; deleted files excluded)
• 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 existing file watcher picks up the write and reloads the diff automatically

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)
• 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 (this dropped every "n" typed in early testing)

Test plan

• pnpm test (oxlint + tsc/vite + cargo test) passes; new Rust test covers write_worktree_file (overwrite, no-create, traversal rejection)
• Verified end-to-end with headless Chromium against a scratch repo: Edit → type → Save writes to disk and the diff refreshes; Edit → type → Cancel reverts the view and leaves disk untouched