fix(harness): 补充 train 模式 run_id 校验 + 拆分函数保持 radon Grade B

- _validate_mode_deps: 恢复 train 非 resume/fresh 时必须提供 run_id 校验
- 提取 _validate_train_run_id 用 early return 展平条件，避免 radon Grade C
- 合并 promote run_id 检查到 diagnose/evolve/promote 统一检查
- 新增 4 个测试：train+run_id / train+resume / train+fresh / train+baseline
- radon cc -n C 无输出（全部 Grade B 或更好）
- 74 个单元测试全部通过

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/unit/test_harness_config.py

@@ -139,7 +139,7 @@ class TestModeValidation:
     def test_all_valid_modes_accepted(self, valid_mode: str) -> None:
         """全部合法 mode 应通过校验（diagnose/evolve 需 run_id）。"""
         overrides: dict = {"mode": valid_mode}
-        if valid_mode in ("diagnose", "evolve"):
+        if valid_mode in ("diagnose", "evolve", "train"):
             overrides["run_id"] = "run-001"
         if valid_mode in ("eval", "promote"):
             overrides["version"] = "v1"
@@ -172,6 +172,27 @@ class TestModeValidation:
         with pytest.raises(ValueError, match="run.id"):
             _validate(cfg)
 
+    def test_train_mode_requires_run_id_without_resume_fresh(self) -> None:
+        """train 模式非 resume/fresh 时必须提供 run_id。"""
+        cfg = _make_config(mode="train", run_id="", resume=False, fresh=False)
+        with pytest.raises(ValueError, match="run_id"):
+            _validate(cfg)
+
+    def test_train_mode_resume_without_run_id_accepted(self) -> None:
+        """train 模式 resume=True 时不需要 run_id。"""
+        cfg = _make_config(mode="train", run_id="", resume=True)
+        _validate(cfg)
+
+    def test_train_mode_fresh_without_run_id_accepted(self) -> None:
+        """train 模式 fresh=True 时不需要 run_id。"""
+        cfg = _make_config(mode="train", run_id="", fresh=True)
+        _validate(cfg)
+
+    def test_train_mode_with_run_id_accepted(self) -> None:
+        """train 模式提供 run_id 时应通过（旧式基线 run）。"""
+        cfg = _make_config(mode="train", run_id="baseline-001")
+        _validate(cfg)
+
     def test_concurrency_positive(self) -> None:
         """concurrency <= 0 应报错。"""
         cfg = _make_config(concurrency=0)