feat: add hook-based idle detection for respawn controller

Implement multi-phase improvement to respawn controller's idle detection:

Phase 1 - Stop Hook Detection:
- Add signalStopHook() method - definitive signal when Claude finishes
- 3s confirmation timer to handle race conditions
- Skip AI check when hook received (100% confidence)

Phase 2 - idle_prompt Detection:
- Add signalIdlePrompt() method - fires after 60s+ of idle
- Immediately confirms idle (no confirmation timer needed)

Phase 3 - Transcript File Monitoring:
- New TranscriptWatcher class watches session JSONL files
- Detects: completion, tool execution, plan mode, errors
- Signals respawn controller for supporting detection

Web UI Updates:
- Hook indicator with purple styling and pulse animation
- Shows "Stop hook received" or "idle_prompt hook received"
- 100% confidence displayed with hook-confirmed style

This significantly improves idle detection reliability by using
definitive signals from Claude Code rather than parsing terminal output.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/web/public/index.html

@@ -73,6 +73,7 @@
         <span class="detection-status" id="detectionStatus"></span>
         <span class="detection-waiting" id="detectionWaiting" style="display: none;"></span>
         <span class="detection-confidence" id="detectionConfidence"></span>
+        <span class="detection-hook" id="detectionHook" style="display: none;"></span>
         <span class="detection-ai-check" id="detectionAiCheck" style="display: none;"></span>
         <span class="respawn-spacer"></span>
         <span class="respawn-cycles">#<span id="respawnCycleCount">0</span></span>