feat(engine): 实现阵营技能树系统

- 新增 FactionSkill、FactionSkillTree、FactionSkills 数据模型
- 创建 faction_skills.py 模块处理技能解锁和效果应用
- 技能解锁条件：power >= cost 且前置技能已解锁
- 技能效果：increase_positive_emotion、reduce_conflict、increase_fear、suppress_others
- 添加 4 个技能树测试用例，全部 30 个测试通过

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

engine-python/app/state.py

@@ -2,6 +2,7 @@ import json
 from pathlib import Path
 from .models import WorldState, AgentState, GlobalMeter, Stance, Factions, FactionData
 from .story_arcs import get_default_story_arcs
+from .faction_skills import get_default_faction_skills
 
 STATE_FILE = Path(__file__).parent.parent / "state.json"
 
@@ -35,6 +36,7 @@ def get_default_state() -> WorldState:
             fearful=FactionData(power=0, threshold=10, skill="panic", members=[])
         ),
         story_arcs=get_default_story_arcs(),
+        faction_skills=get_default_faction_skills(),
     )