Sprint 6G: operator chat with assistant response mode

[samrusani]

Summary

• turn /chat into a dual-mode operator surface with assistant-response mode and retained governed-request mode
• add bounded response history, explicit mode switching, and fixture fallback when API configuration is absent
• keep the sprint inside the shipped /v0/responses and /v0/approvals/requests seams with no broader scope 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 within the intended /chat UI scope over the shipped /v0/responses and /v0/approvals/requests seams
• no backend, Gmail, Calendar, auth, runner, or broader scope expansion entered the sprint