- 3 new DB tables: subtasks, task_collaboration, user_groups
- Subtasks CRUD with auto-complete parent when all done
- Collaboration requests: assign, transfer, collaborate, claim
- Claim = instant, others need approval (accept/reject)
- Assign to whole user group at once
- Workload tracking: per-user active/completed/pending
- User groups (teams) CRUD with member management
- 39/39 tests passed
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
