feat(chat): surface unseen session activity

[kevmyung]

Summary

• persist durable attention cursors when asynchronous assistant completions are committed
• expose unseen activity in the session list, poll it in the sidebar, and mark it seen only after the result is rendered
• show a minimal activity dot for inactive sessions and reset attention state on truncate

Validation

• npm test (59 files, 731 tests)
• npm run type-check
• pytest tests/unit/test_mailbox.py -q (21 passed)
• dev deployment: frontend ECS revision 72, orchestrator v52, general subagent v11
• dev E2E: real research job reached delivered, session became hasUnseenUpdate: true, then cleared after the seen API call