fix: build 命令在 plan 未完成时给出友好提示而非未预期错误

tests/core/assembler.test.ts

@@ -184,10 +184,15 @@ describe("assembleBuildPrompt", () => {
     expect(prompt).toContain("归档");
   });
 
-  it("task.md 不存在时抛出错误", async () => {
-    await expect(
-      assembleBuildPrompt(defaultConfig, TMP_DIR, "nonexistent"),
-    ).rejects.toThrow("task.md not found");
+  it("task.md 不存在时抛出 CommandError 并附带提示", async () => {
+    try {
+      await assembleBuildPrompt(defaultConfig, TMP_DIR, "nonexistent");
+      expect.unreachable();
+    } catch (e: any) {
+      expect(e.message).toContain("尚未完成规划");
+      expect(e.message).toContain("nonexistent");
+      expect(e.hint).toContain("rune plan");
+    }
   });
 });