feat(types): extend GeneratedQuestion with skill_target & difficulty_steps

- Add skill_target (str | None) and difficulty_steps (int | None) fields
  to GeneratedQuestion dataclass with field(default=None)
- Update loader.py to pass new fields from JSON (backward-compatible)
- Update pools.py _q_to_dict/_dict_to_q for serialization compat
- Add question_gen_v2 config section to default.yaml
- Add comprehensive test coverage (7 tests)

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

app/question_gen/loader.py

@@ -46,6 +46,8 @@ def load_benchmark(questions_dir: Path) -> list[GeneratedQuestion]:
                     answer=qa["answer"],
                     source_nodes=tuple(qa.get("source_nodes", ())),
                     difficulty=qa.get("difficulty", _LEGACY_DEFAULT_DIFFICULTY),
+                    skill_target=qa.get("skill_target"),
+                    difficulty_steps=qa.get("difficulty_steps"),
                 )
             )
     return results