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/engine.py

@@ -11,6 +11,7 @@ from .factions import (
     update_factions, apply_faction_influence, check_and_trigger_faction_event
 )
 from .voting import process_votes, apply_votes_to_factions
+from .faction_skills import check_and_unlock_skills, apply_skill_effects
 
 MAX_EVENTS = 20
 MAX_MEMORY_PER_AGENT = 3
@@ -106,6 +107,12 @@ def process_events(state: WorldState, events: List[Event]) -> WorldState:
     # 派系影响世界情绪
     apply_faction_influence(state)
 
+    # 检查并解锁技能
+    check_and_unlock_skills(state)
+
+    # 应用已解锁技能效果
+    apply_skill_effects(state)
+
     return state