fix(config): debounce fsnotify reload events to avoid mid-write reads

os.WriteFile truncates then writes, so inotify can deliver back-to-back
MODIFY events while the file is momentarily empty. Reading mid-sequence
made TestStoreReloadsWhenLocalOverrideIsCreated and
TestStoreReloadsConfigAndKeepsPreviousOnInvalidYAML flake on Linux CI by
firing onReload with a blank config before the write settled.

Coalesce updates over a 50ms quiet window before reloading, and skip
onReload when the diff is empty so subscribers only see real changes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/internal/shared/config/store.go

@@ -103,11 +103,40 @@ func (s *Store) Watch(ctx context.Context, logger *zap.Logger, onReload func(Rel
 		case <-s.ctx.Done():
 			return nil
 		case <-s.updates:
+			if !s.coalesceUpdates(ctx, reloadDebounce) {
+				return nil
+			}
 			s.reload(logger, onReload)
 		}
 	}
 }
 
+// reloadDebounce coalesces fsnotify events from a single write. os.WriteFile
+// opens with O_CREATE|O_TRUNC then writes, so inotify can deliver back-to-back
+// MODIFY events with the file momentarily empty. Reading mid-sequence yields a
+// blank config; waiting for a brief quiet period lets the write settle.
+const reloadDebounce = 50 * time.Millisecond
+
+func (s *Store) coalesceUpdates(ctx context.Context, window time.Duration) bool {
+	timer := time.NewTimer(window)
+	defer timer.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return false
+		case <-s.ctx.Done():
+			return false
+		case <-s.updates:
+			if !timer.Stop() {
+				<-timer.C
+			}
+			timer.Reset(window)
+		case <-timer.C:
+			return true
+		}
+	}
+}
+
 func (s *Store) Close() {
 	if s == nil {
 		return
@@ -138,6 +167,9 @@ func (s *Store) reload(logger *zap.Logger, onReload func(ReloadEvent)) {
 	s.mu.Unlock()
 
 	changes := Diff(previous, next)
+	if len(changes) == 0 {
+		return
+	}
 	if logger != nil {
 		logger.Info("config reloaded",
 			zap.Strings("files", files),