fix(mcp): use strcpy for semantic result strings to prevent use-after-free

The semantic_results section used yyjson_mut_obj_add_str (borrows pointer)
then called cbm_node_free_fields which freed those strings. The yyjson doc
then held dangling pointers, producing garbage in the JSON output.

Fix: use yyjson_mut_obj_add_strcpy (copies string) for all node fields
in the vector-only result loop.

Author: Your Name

src/mcp/mcp.c

@@ -1140,14 +1140,14 @@ static char *handle_search_graph(cbm_mcp_server_t *srv, const char *args) {
                 if (cbm_store_find_node_by_id(store, rrf_results[i].node_id,
                                               &vnode) == CBM_STORE_OK) {
                     yyjson_mut_val *vitem = yyjson_mut_obj(doc);
-                    yyjson_mut_obj_add_str(doc, vitem, "name",
-                                           vnode.name ? vnode.name : "");
-                    yyjson_mut_obj_add_str(doc, vitem, "qualified_name",
-                                           vnode.qualified_name ? vnode.qualified_name : "");
-                    yyjson_mut_obj_add_str(doc, vitem, "label",
-                                           vnode.label ? vnode.label : "");
-                    yyjson_mut_obj_add_str(doc, vitem, "file_path",
-                                           vnode.file_path ? vnode.file_path : "");
+                    yyjson_mut_obj_add_strcpy(doc, vitem, "name",
+                                              vnode.name ? vnode.name : "");
+                    yyjson_mut_obj_add_strcpy(doc, vitem, "qualified_name",
+                                              vnode.qualified_name ? vnode.qualified_name : "");
+                    yyjson_mut_obj_add_strcpy(doc, vitem, "label",
+                                              vnode.label ? vnode.label : "");
+                    yyjson_mut_obj_add_strcpy(doc, vitem, "file_path",
+                                              vnode.file_path ? vnode.file_path : "");
                     yyjson_mut_obj_add_real(doc, vitem, "similarity",
                                            rrf_results[i].similarity);
                     yyjson_mut_obj_add_real(doc, vitem, "rrf_score",