docs: document scheduled recall system ...

- Add docs/recall.md with full architecture, data model, tools, loop behavior
- Update docs/api.md with recall endpoints (GET/DELETE/POST recall) and recall_update WS event
- Update docs/websocket.md with recall_update frame and session_sync case #3
- Update docs/tools.md: add ScheduleRecallTool and ManageRecallTool
- Update docs/sessions.md: add is_recall message flag
- Update docs/index.md: link recall.md in file map

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>