Implement review-fix-loop local locking, isolated attempts, and recovery

[shaug]

Outcome

Implement the local execution substrate for review-fix-loop: common-directory locking, isolated attempts, durable checkpoints, recovery, and clean canonical promotion.

Parent epic: #95
Design: design/review-fix-loop.md

Scope

• Acquire stable local-ref locks, plus the remote-target lock required by update_pr, in deterministic order.
• Create and clean isolated attempt worktrees without mutating user-specified reference branches.
• Persist enough state to resume or safely report interruption.
• Promote a converged candidate only after verifying the canonical worktree is clean and the expected old state still matches.
• Preserve recoverable commits and report their identifiers when promotion cannot complete.

Non-goals

• A distributed lease, ownership, fencing, or coordinator service.
• Review orchestration or finding selection.
• Remote publication.

Acceptance criteria

• Conflicting local invocations cannot both own the same target.
• Lock ordering avoids self-induced deadlocks for multi-target update_pr work.
• Attempts occur outside the canonical worktree and leave it unchanged until promotion.
• Interrupted attempts can be recovered or reported without losing commits.
• Dirty or advanced canonical state fails closed and preserves the candidate.
• Cleanup never deletes user-owned work or reference branches.

Validation

• Deterministic tests for contention, interruption, stale state, dirty worktrees, promotion races, and cleanup.
• Repository just format, just lint, and just test.

Dependencies

Blocked by the contract leaf.

[shaug]

Closing manually: delivered and merged via #109 (squash commit 26b4cf47168dc8432f7d6e5e4597439af6391a51 on main), with acceptance evidence verified independently after merge.

Acceptance criteria (all pre-merge, all pass)

• Conflicting local invocations cannot both own the same target — non-blocking flock-backed lock keyed by Git common directory and ref (LockingTests).
• Lock ordering avoids self-induced deadlocks for multi-target update_pr work — local-ref lock always acquired before the remote-target lock, released in reverse; every acquisition is non-blocking, so no invocation can ever wait on another.
• Attempts occur outside the canonical worktree and leave it unchanged until promotion — create_attempt uses git worktree add into a dedicated path/branch (AttemptLifecycleTests).
• Interrupted attempts can be recovered or reported without losing commits — recover_interrupted_attempts (RecoveryTests), including ambiguous-state rejection rather than silent guessing.
• Dirty or advanced canonical state fails closed and preserves the candidate — promote_attempt's precondition checks (DirtyWorktreeError, StaleBaseError).
• Cleanup never deletes user-owned work or reference branches — cleanup_attempt refuses any branch outside the review-fix-loop/attempt/ namespace (UnsafeCleanupError).

Validation

• python3 -m unittest discover -s skills/review-fix-loop/scripts/tests -p 'test_*.py' — 154 tests, OK (run repeatedly pre-merge with no flakes; reconfirmed against the merged main head 26b4cf4 post-merge).
• just format, just lint, just test — all pass at the merged head.

Review

Fresh, independent, read-only review-code-change pass (raw evidence only) returned aggregate verdict clean for the exact merged candidate content (tree-identical to the squash commit). One defer-severity, non-gating correctness observation was recorded and intentionally left for the loop-assembly child: this module does not itself reread the comparison-base ref during a live remediation cycle (only at resume) — driving repeated cycles and rebuilding evidence after base drift requires reviewer orchestration, which is #98's territory and this ticket's explicit non-goal.

Post-merge verification

• Merge commit 26b4cf47168dc8432f7d6e5e4597439af6391a51 is on main; local mainline worktree fast-forwarded and confirmed.
• Squash commit tree (43dbaa000ce1a5fda5358bba430e8b4c56f0b925) is byte-identical to the reviewed PR head's tree — full patch equivalence.
• Remote and local feature branches deleted after verified merge.

No post-merge acceptance items were required for this ticket.