- Add findReusableHeartbeatTask() to find most recent done/in_review heartbeat task per agent
- Add cleanupDuplicateHeartbeatTasks() to cancel old duplicates using ROW_NUMBER window function
- Add 12 unit tests covering coalescing behavior, cross-agent isolation, status filtering
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>