refactor: loadConfig 统一走 mergeConfig 路径

tests/core/config.test.ts

@@ -75,6 +75,17 @@ describe("loadConfig", () => {
     expect(config.stages.plan!.documents[0].name).toBe("spec");
   });
 
+  it("空配置 stages: {} 时返回完整默认配置", async () => {
+    const runeDir = join(TMP_DIR, ".rune");
+    await mkdir(runeDir, { recursive: true });
+    await writeFile(join(runeDir, "config.yaml"), "stages: {}");
+    const config = await loadConfig(TMP_DIR);
+    expect(config.stages.discuss).toBeDefined();
+    expect(config.stages.plan).toBeDefined();
+    expect(config.stages.build).toBeDefined();
+    expect(config.stages.archive).toBeDefined();
+  });
+
   it("YAML 解析错误时返回默认配置", async () => {
     const runeDir = join(TMP_DIR, ".rune");
     await mkdir(runeDir, { recursive: true });