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/models.py

@@ -66,6 +66,25 @@ class Votes(BaseModel):
     voted_users: List[str] = Field(default_factory=list)
 
 
+class FactionSkill(BaseModel):
+    """单个技能"""
+    unlocked: bool = False
+    cost: int = 10
+    effect: str = ""
+    requires: List[str] = Field(default_factory=list)
+
+
+class FactionSkillTree(BaseModel):
+    """单个阵营的技能树"""
+    skills: Dict[str, FactionSkill] = Field(default_factory=dict)
+
+
+class FactionSkills(BaseModel):
+    """所有阵营的技能树"""
+    optimists: FactionSkillTree = Field(default_factory=FactionSkillTree)
+    fearful: FactionSkillTree = Field(default_factory=FactionSkillTree)
+
+
 class FactionEventResult(BaseModel):
     """阵营事件触发结果"""
     type: Optional[str] = None  # "festival" | "panic" | None
@@ -113,6 +132,7 @@ class WorldState(BaseModel):
     factions: Factions = Field(default_factory=Factions)
     story_arcs: Dict[str, StoryArc] = Field(default_factory=dict)
     votes: Votes = Field(default_factory=Votes)
+    faction_skills: FactionSkills = Field(default_factory=FactionSkills)
 
 
 class Event(BaseModel):