fix: enable hot reload for amp-model-mappings config

- Store ampModule in Server struct to access it during config updates
- Call ampModule.OnConfigUpdated() in UpdateClients() for hot reload
- Watch config directory instead of file to handle atomic saves (vim, VSCode, etc.)
- Improve config file event detection with basename matching
- Add diagnostic logging for config reload tracing

internal/api/modules/amp/amp.go

@@ -166,7 +166,10 @@ func (m *AmpModule) getAuthMiddleware(ctx modules.Context) gin.HandlerFunc {
 func (m *AmpModule) OnConfigUpdated(cfg *config.Config) error {
 	// Update model mappings (hot-reload supported)
 	if m.modelMapper != nil {
+		log.Infof("amp config updated: reloading %d model mapping(s)", len(cfg.AmpModelMappings))
 		m.modelMapper.UpdateMappings(cfg.AmpModelMappings)
+	} else {
+		log.Warnf("amp model mapper not initialized, skipping model mapping update")
 	}
 
 	if !m.enabled {

internal/api/server.go

@@ -150,6 +150,9 @@ type Server struct {
 	// management handler
 	mgmt *managementHandlers.Handler
 
+	// ampModule is the Amp routing module for model mapping hot-reload
+	ampModule *ampmodule.AmpModule
+
 	// managementRoutesRegistered tracks whether the management routes have been attached to the engine.
 	managementRoutesRegistered atomic.Bool
 	// managementRoutesEnabled controls whether management endpoints serve real handlers.
@@ -268,14 +271,14 @@ func NewServer(cfg *config.Config, authManager *auth.Manager, accessManager *sdk
 	s.setupRoutes()
 
 	// Register Amp module using V2 interface with Context
-	ampModule := ampmodule.NewLegacy(accessManager, AuthMiddleware(accessManager))
+	s.ampModule = ampmodule.NewLegacy(accessManager, AuthMiddleware(accessManager))
 	ctx := modules.Context{
 		Engine:         engine,
 		BaseHandler:    s.handlers,
 		Config:         cfg,
 		AuthMiddleware: AuthMiddleware(accessManager),
 	}
-	if err := modules.RegisterModule(ctx, ampModule); err != nil {
+	if err := modules.RegisterModule(ctx, s.ampModule); err != nil {
 		log.Errorf("Failed to register Amp module: %v", err)
 	}
 
@@ -916,6 +919,16 @@ func (s *Server) UpdateClients(cfg *config.Config) {
 		s.mgmt.SetAuthManager(s.handlers.AuthManager)
 	}
 
+	// Notify Amp module of config changes (for model mapping hot-reload)
+	if s.ampModule != nil {
+		log.Debugf("triggering amp module config update")
+		if err := s.ampModule.OnConfigUpdated(cfg); err != nil {
+			log.Errorf("failed to update Amp module config: %v", err)
+		}
+	} else {
+		log.Warnf("amp module is nil, skipping config update")
+	}
+
 	// Count client sources from configuration and auth directory
 	authFiles := util.CountAuthFiles(cfg.AuthDir)
 	geminiAPIKeyCount := len(cfg.GeminiKey)