fix: trigger initial session time threshold

[GautamSharma99]

Summary

Fixes time-based automatic memory updates for new agent sessions.

Previously, each hook reset last_seen_at before the worker checked the time threshold, making the elapsed time effectively zero. New
sessions could therefore only trigger updates through the Stop-count threshold.

Changes

• Added persistent first_seen_at session tracking.
• Use first_seen_at for the initial time threshold.
• Preserve existing post-memory-update and grace-period behavior.
• Backfill existing sessions from last_seen_at.
• Added regression and migration coverage.

Testing

npm test
npm run typecheck