Sprint 6H: thread and session continuity APIs

[samrusani]

Summary

• add narrow user-scoped continuity APIs for thread create, thread list/detail, thread sessions, and immutable thread events
• keep the sprint limited to the existing continuity store and deterministic continuity review behavior
• add unit and Postgres-backed integration coverage for create, ordering, shape, isolation, and not-found behavior

Verification

• ./.venv/bin/python -m pytest tests/unit: PASS
• ./.venv/bin/python -m pytest tests/integration: PASS

Notes

• the sprint stayed limited to the thread/session continuity API seam
• no UI, Gmail, Calendar, task, approval, execution, auth, or runner scope expansion entered the sprint