Sprint 6I: chat thread selection and continuity review UI

[samrusani]

Summary

• extend /chat with thread selection, compact thread creation, and bounded continuity review using the shipped continuity APIs
• keep assistant-response and governed-request modes explicit while anchoring both to selected-thread state instead of manual raw thread-id entry
• keep the sprint inside the shipped continuity and chat seams with no backend or broader workflow expansion

Verification

• npm run lint in apps/web: PASS
• npm test in apps/web: PASS
• npm run build in apps/web: PASS

Notes

• the sprint stayed a UI sprint over the shipped continuity and chat seams
• no backend, Gmail, Calendar, auth, runner, or broader workflow-scope expansion entered the sprint