스킬 시스템 구현

- 애니메이션 이벤트 기반 스킬 시스템 추가
  - SkillData: 스킬 데이터 (클립, 쿨타임, 효과 목록)
  - SkillController: 스킬 실행 및 애니메이션 제어
  - AnimatorOverrideController로 단일 State에서 다양한 스킬 재생

- 스킬 효과 시스템
  - DamageEffect, HealEffect, BuffEffect
  - KnockbackEffect, SoundEffect, SpawnEffect
  - 범위 공격 및 팀 구분 지원

- Team 컴포넌트로 아군/적 구분

- 스킬 중 이동 제한
  - IsPlayingAnimation으로 애니메이션 종료까지 이동 불가
  - OnSkillEnd 호출 시 다음 스킬 시전 가능

- 입력 시스템에 스킬 슬롯 6개 추가

- 애니메이션 에셋 추가 및 정리
  - AnimationSwordCombat 패키지 추가 (검 공격 애니메이션)
  - PlayerAnimationController에 Skill 상태 추가
  - External_Used 폴더 구조 정리

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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