Add session monitoring and lifecycle improvements to the gateway:
- Session metrics: createdAt, promptCount, durationMs tracking
- Channel tracking: which channels (websocket, discord) are connected
- REST API at /api/sessions for listing, inspecting, and destroying sessions
- Fix piSession.dispose() call in destroySession (FIX-01)
- WebSocket gateway now tracks channel connections per session
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>