fix: atomic writes for manifest/record_run/update_best (tmp+replace)

tests/unit/test_harness_workspace.py

@@ -259,6 +259,33 @@ def test_update_manifest_valid(store_dir: Path, workspace_dir: Path) -> None:
     assert manifest["current"]["skills"] == "skills/v2"
 
 
+def test_update_manifest_is_atomic(
+    store_dir: Path, workspace_dir: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    """写 manifest 途中崩溃不产生半截 JSON（原子写：os.replace 失败也不损原文件）。"""
+    from app.harness import workspace as ws
+
+    init_workspace(
+        workspace_dir,
+        store_dir,
+        questions="benchmarks/Video-MME",
+        skills_version="v1",
+        prompts_version="v1",
+    )
+    original = (workspace_dir / "manifest.json").read_text()
+
+    def _boom(_src: object, _dst: object) -> None:
+        raise OSError("crash during replace")
+
+    monkeypatch.setattr(ws.os, "replace", _boom)
+    with pytest.raises(OSError, match="crash during replace"):
+        update_manifest(workspace_dir, skills="skills/v2")
+
+    # 原 manifest 未被破坏（内容不变且仍是合法 JSON）
+    assert (workspace_dir / "manifest.json").read_text() == original
+    assert json.loads((workspace_dir / "manifest.json").read_text())
+
+
 # ---------------------------------------------------------------------------
 # record_run
 # ---------------------------------------------------------------------------