Copilot should clearly signal when all review tasks are complete

[mgradwohl]

When I assign multiple code review tasks to Copilot using @copilot,
there's no clear way to know when it's finished with ALL of them.

Current behavior:

• I have to manually check each comment thread
• I have to guess if silence means "done" or "still working"
• No summary or notification when complete

Desired behavior:

• A clear "All tasks complete" notification or comment
• A status indicator showing progress (e.g., "2/3 tasks done")
• A final summary linking to all commits made

This would dramatically improve the UX and make it clear when
Copilot hands off back to the human developer.

┌─────────────────────────────────────────┐
│ ✅ Copilot Review Tasks Complete        │
├─────────────────────────────────────────┤
│ ✅ Upgrade artifacts to v6 (0071a19)    │
│ ✅ Fix PIPESTATUS (already applied)     │
│ ✅ Use POSIX = comparison (abc123d)     │
├─────────────────────────────────────────┤
│ 🎯 Ready for your review!                │
│                                         │
│ Next steps:                              │
│ 1. Review the changes                   │
│ 2. Verify CI passes                     │
│ 3. Mark as "Ready for review"           │
│ 4. Merge when ready                     │
└─────────────────────────────────────────┘