refactor: loadConfig 统一走 mergeConfig 路径

src/core/config.ts

@@ -25,12 +25,9 @@ export async function loadConfig(projectRoot: string): Promise<RuneConfig> {
   try {
     const content = await readFile(configPath, "utf-8");
     const userConfig = parseYaml(content) as Partial<RuneConfig> | null;
-    if (!userConfig?.stages) {
-      return structuredClone(defaultConfig);
-    }
-    return mergeConfig(userConfig);
+    return mergeConfig(userConfig ?? {});
   } catch {
-    return structuredClone(defaultConfig);
+    return mergeConfig({});
   }
 }