feat(question_gen): add generate-v2 CLI subcommand and experiment script

- Add generate-v2 subparser with --config, --store-dir, --db-path,
  --seed, and --dry-run arguments to tools/generate_questions.py
- Implement _run_generate_v2 async handler: config loading, video
  discovery, DI client construction, TreeIndex loading, pipeline
  invocation, and result persistence
- Add scripts/generate_questions_v2.sh following build_trees.sh
  conventions (source .env, conda run python path, MODE=mock support)
- Update app/question_gen/__init__.py to export full v2 public API:
  run_pipeline_v2, PipelineConfig, PipelineResult, QuestionFamilySpec,
  ALL_FAMILIES, CandidateQuestion, generate_one_v2, GateReport, run_gates
- Add QuestionGenStore.load_progress() for pipeline resumption
- Add integration tests for CLI help and dry-run behavior
- Update test_question_gen_api to match expanded __all__

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

tests/unit/test_question_gen_api.py

@@ -35,13 +35,24 @@ class TestQuestionGenPublicAPI:
         assert hasattr(mod, "stratified_sample")
 
     def test_all_exports(self) -> None:
-        """__all__ 包含预期的公开 API（loader + synthesizer）。"""
+        """__all__ 包含预期的公开 API（v1 + v2）。"""
         mod = importlib.import_module("app.question_gen")
         assert set(mod.__all__) == {
+            # v1 接口
             "load_benchmark",
             "stratified_sample",
             "TASK_TYPE_LEVEL_MAP",
             "AnchorContext",
             "generate_one",
             "sample_anchor",
+            # v2 接口
+            "run_pipeline_v2",
+            "PipelineConfig",
+            "PipelineResult",
+            "QuestionFamilySpec",
+            "ALL_FAMILIES",
+            "CandidateQuestion",
+            "generate_one_v2",
+            "GateReport",
+            "run_gates",
         }