Replace scattered _runs + _busy_sessions + _session_sockets with a
single _sessions: dict[str, SessionState] on the orchestrator.

- SessionState dataclass holds run, busy_event, and websockets
- _session_sockets module-level global removed from websocket.py;
  socket tracking moved into orchestrator (add/remove_websocket)
- Event bus subscriber _on_recall_update moved into orchestrator
- Per-session asyncio.Lock added to protect concurrent-run guard
- _cleanup() auto-removes empty SessionState entries

Tests updated to reference _sessions instead of legacy _runs.

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