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>

PLAN.md

@@ -117,14 +117,14 @@ This lets shells build realistic prompts (`username@hostname:~$`) and log activi
 
 Goal: Detect likely-human sessions and make the system smarter.
 
-### 2.1 Human Detection Scoring
+### 2.1 Human Detection Scoring ✅
 - Keystroke timing analysis
 - Track backspace, tab, arrow key usage
 - Command diversity scoring
 - Compute per-session human score, store in sessions table
 - Flag sessions above configurable threshold
 
-### 2.2 Notifications
+### 2.2 Notifications ✅
 - Webhook support (generic HTTP POST, works with Slack/Discord/ntfy)
 - Trigger on: human score threshold crossed, new session started, configurable
 - Include session details in payload