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>

core/types.py

@@ -2,7 +2,7 @@
 
 from __future__ import annotations
 
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 
 
 @dataclass(frozen=True)
@@ -41,6 +41,8 @@ class GeneratedQuestion:
         answer: 正确答案字母（如 "B"）。
         source_nodes: 来源节点 ID 元组。
         difficulty: 难度等级。
+        skill_target: 目标技能标识（v2 出题管线使用，None 表示未指定）。
+        difficulty_steps: 推理步数估计（v2 出题管线使用，None 表示未指定）。
     """
 
     question_id: str
@@ -51,3 +53,5 @@ class GeneratedQuestion:
     answer: str
     source_nodes: tuple[str, ...]
     difficulty: str
+    skill_target: str | None = field(default=None)
+    difficulty_steps: int | None = field(default=None)