fix(mcp): map tool-not-found to CodeNotFound via registry sentinel

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/mcp/server.go

@@ -90,7 +90,7 @@ func (s *Server) handlePOST(w http.ResponseWriter, r *http.Request) {
 		out, err := s.opts.Registry.Dispatch(r.Context(), p.Name, p.Arguments)
 		if err != nil {
 			code := -32000
-			if errors.Is(err, ErrToolNotFound) {
+			if errors.Is(err, registry.ErrToolNotFound) {
 				code = CodeNotFound
 			}
 			writeJSON(w, http.StatusOK,
@@ -125,8 +125,6 @@ func (s *Server) handleGET(w http.ResponseWriter, r *http.Request) {
 	<-r.Context().Done()
 }
 
-var ErrToolNotFound = errors.New("tool not found")
-
 func writeJSON(w http.ResponseWriter, status int, v any) {
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(status)