Fix FileWatcher robustness issues (bd-71)

- Fix Debouncer race condition with sequence numbers to prevent double-fire
- Add parent directory watch to catch file creates/renames
- Add .git/HEAD watch for branch change detection
- Implement retry/backoff (50-400ms) for re-establishing JSONL watch
- Handle Create/Chmod events in addition to Write
- Add .git/HEAD polling in polling mode
- All 18 debouncer and watcher tests pass

Amp-Thread-ID: https://ampcode.com/threads/T-4029d643-b4b4-4d3b-bd85-74461f78cd7f
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/daemon_debouncer.go

@@ -12,6 +12,7 @@ type Debouncer struct {
 	timer    *time.Timer
 	duration time.Duration
 	action   func()
+	seq      uint64 // Sequence number to prevent stale timer fires
 }
 
 // NewDebouncer creates a new debouncer with the given duration and action.
@@ -34,11 +35,21 @@ func (d *Debouncer) Trigger() {
 		d.timer.Stop()
 	}
 
+	// Increment sequence number to invalidate any pending timers
+	d.seq++
+	currentSeq := d.seq
+
 	d.timer = time.AfterFunc(d.duration, func() {
-		d.action()
 		d.mu.Lock()
-		d.timer = nil
-		d.mu.Unlock()
+		defer d.mu.Unlock()
+
+		// Only fire if this is still the latest trigger
+		if d.seq == currentSeq {
+			d.timer = nil
+			d.mu.Unlock() // Unlock before calling action to avoid holding lock during callback
+			d.action()
+			d.mu.Lock() // Re-lock for defer
+		}
 	})
 }