Add review functionality:
- GET /tasks/:id/review shows review interface
- Look up commit by Task-Id: trailer in git log (same logic as Jr.reviewTask)
- Display git diff in a pre/code block
- Accept button: POST /tasks/:id/accept -> marks Done, redirects to /tasks/:id
- Reject button with notes textarea: POST /tasks/:id/reject -> marks Open with retry context
- If no commit found, show message explaining task may not be complete
- Handle merge conflict detection (reuse checkMergeConflict logic)