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/styles.css

@@ -671,6 +671,31 @@ body {
   color: var(--yellow);
 }
 
+.detection-confidence.hook-confirmed {
+  background: rgba(139, 92, 246, 0.3);
+  color: var(--purple, #a78bfa);
+}
+
+.detection-hook {
+  font-size: 0.7rem;
+  padding: 0.1rem 0.4rem;
+  border-radius: 3px;
+  background: rgba(139, 92, 246, 0.15);
+  color: var(--purple, #a78bfa);
+}
+
+.detection-hook.hook-active {
+  background: rgba(139, 92, 246, 0.25);
+  color: var(--purple, #a78bfa);
+  font-weight: 500;
+  animation: pulse-hook 1.5s ease-in-out infinite;
+}
+
+@keyframes pulse-hook {
+  0%, 100% { opacity: 1; }
+  50% { opacity: 0.7; }
+}
+
 @keyframes spin {
   to { transform: rotate(360deg); }
 }