Added 'done' status to task lifecycle for verification period before completion.
**Changes:**
- Added 'done' to task_status enum (pending, open, in_progress, testing, blocked, done, completed)
- Added auto_close_at timestamp column for scheduling auto-close
- Created index for efficient cron job queries
**Workflow:**
1. Task marked 'done' → auto_close_at = NOW() + 7 days
2. Cron job runs daily to check tasks past auto_close_at
3. If no related issues → auto-close to 'completed'
4. If related issues → keep as 'done' (prevents premature closure)
**Lifecycle:**
open → in_progress → done (7-day verification) → completed (auto-closed)
task:CF-454
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
