fix: fail loud on diag model fingerprint drift vs .env SEARCH_LLM_MODEL

Codex CHANGES_REQUESTED 复审：
- Critical：diag_fingerprint 用 config.diag.model，但 Phase 1 诊断 LLM 从 .env
  SEARCH_LLM_MODEL 构建，两者不一致会让信号以错误模型指纹落库，破坏可复现/
  resume/隔离。build_diagnosis_deps 新增 expected_model 参数，Phase 1 执行前
  fail loud 校验 config.model == settings.search_llm_model（附两值）。
- Minor：config/video_split.yaml diag.model 注释由 JUDGE_LLM_MODEL 更正为
  SEARCH_LLM_MODEL，与实现对齐。
- 补两个单测：模型不一致 fail loud + 缺凭证 fail loud。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/unit/test_video_split_cli.py

@@ -161,6 +161,42 @@ def test_dry_run_computes_fingerprint_without_llm(monkeypatch, tmp_path, capsys)
     assert not (tmp_path / "_dry_run_signals.db").exists()
 
 
+def test_build_diagnosis_deps_model_mismatch_fails_loud(monkeypatch, tmp_path):
+    """config.diag.model 与 .env SEARCH_LLM_MODEL 不一致 → fail loud（指纹漂移防护）。"""
+
+    class _FakeSettings:
+        search_llm_model = "actual-model-in-env"
+        search_llm_base_url = "https://api.example"
+        search_llm_api_key = "sk-xxx"
+
+    monkeypatch.setattr(cli, "_DiagLLMSettings", lambda: _FakeSettings())
+    with pytest.raises(SystemExit) as exc:
+        cli.build_diagnosis_deps(
+            harness_db=tmp_path / "h.db",
+            concurrency=2,
+            expected_model="deepseek-v4-pro",  # 与 env 不一致
+        )
+    # 报错须同时暴露两个值，便于人对齐
+    msg = str(exc.value)
+    assert "deepseek-v4-pro" in msg
+    assert "actual-model-in-env" in msg
+
+
+def test_build_diagnosis_deps_missing_credentials_fails_loud(monkeypatch, tmp_path):
+    """.env 缺 search LLM 凭证 → fail loud（先于模型一致性校验）。"""
+
+    class _EmptySettings:
+        search_llm_model = ""
+        search_llm_base_url = ""
+        search_llm_api_key = ""
+
+    monkeypatch.setattr(cli, "_DiagLLMSettings", lambda: _EmptySettings())
+    with pytest.raises(SystemExit):
+        cli.build_diagnosis_deps(
+            harness_db=tmp_path / "h.db", concurrency=2, expected_model="deepseek-v4-pro"
+        )
+
+
 def test_git_short_sha_nonempty():
     """仓库内 git_short_sha 返回非空短 SHA。"""
     sha = cli.git_short_sha()