feat: add human detection scoring and webhook notifications

Implement phase 2.1 (human detection) and 2.2 (notifications):

- Detection scorer computes 0.0-1.0 human likelihood from keystroke
  timing variance, special key usage, typing speed, command diversity,
  and session duration
- Webhook notifier sends JSON POST to configured endpoints with
  deduplication, custom headers, and event filtering
- RecordingChannel gains an event callback for feeding keystrokes
  to the scorer without coupling shell and detection packages
- Server wires scorer into session lifecycle with periodic updates
  and threshold-based notification triggers
- Web UI shows human score in session tables with highlighting
- New config sections: [detection] and [[notify.webhooks]]

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -42,6 +42,10 @@ Key settings:
 - `shell.fake_user` — override username in prompt; empty uses the authenticated user
 - `web.enabled` — enable the web dashboard (default `false`)
 - `web.listen_addr` — web dashboard listen address (default `:8080`)
+- `detection.enabled` — enable human detection scoring (default `false`)
+- `detection.threshold` — score threshold (0.0–1.0) for flagging sessions (default `0.6`)
+- `detection.update_interval` — how often to recompute scores (default `5s`)
+- `notify.webhooks` — list of webhook endpoints for notifications (see example config)
 
 ### Run