feat(pool): squash merge + persist review to SQLite

[robotlearning123]

Summary

• Squash merge: release(name, true, taskId) squash-merges agent commits into a single result commit on main (task({id}): {summary}). Full agent history preserved at refs/tasks/{taskId}. Without taskId, falls back to legacy ff/merge for backward compat.
• Review persistence: task.review (ReviewResult) persisted to SQLite via new review TEXT column. Round-trips correctly through save/get/update.
• Types: Added ReviewResult interface and review?: ReviewResult to Task.

Key design decisions

• Uses temp branch + update-ref pattern because git worktrees can't checkout main directly (it's already checked out by the parent repo)
• refs/tasks/{id} only created after successful merge (no orphaned refs on conflict)
• Extracted cleanupTmpBranch() helper to eliminate 4x copy-paste

Known follow-up

• Duplicate ReviewResult interface exists in agent-runner.ts (lacks approve field). Should unify with types.ts version in a follow-up PR.

Test plan

• npx tsc --noEmit — clean
• npm test — 290/290 pass (0 fail)
• New test: squash merge produces exactly 1 commit on main, refs/tasks/{id} preserves all agent commits
• New test: legacy merge (no taskId) preserves all individual commits
• New test: review field round-trips through save/get
• New test: task without review → undefined after load
• New test: update() can set review field

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