chore: Assets 디렉토리 구조 정리 및 네이밍 컨벤션 적용

- Assets/_Game/ 하위로 게임 에셋 통합
- External/ 패키지 벤더별 분류 (Synty, Animations, UI)
- 에셋 네이밍 컨벤션 확립 및 적용
  (Data_Skill_, Data_SkillEffect_, Prefab_, Anim_, Model_, BT_ 등)
- pre-commit hook으로 네이밍 컨벤션 자동 검사 추가
- RESTRUCTURE_CHECKLIST.md 작성

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

Assets/DefaultNetworkPrefabs.asset

@@ -15,7 +15,7 @@ MonoBehaviour:
   IsDefault: 1
   List:
   - Override: 0
-    Prefab: {fileID: 6473031571298860035, guid: 9f538e60b8b98634b8952310b91dfba0, type: 3}
+    Prefab: {fileID: 100000, guid: 0cd47707143f91346868bfc349e4c4e0, type: 3}
     SourcePrefabToOverride: {fileID: 0}
     SourceHashToOverride: 0
     OverridingTargetPrefab: {fileID: 0}
@@ -25,7 +25,7 @@ MonoBehaviour:
     SourceHashToOverride: 0
     OverridingTargetPrefab: {fileID: 0}
   - Override: 0
-    Prefab: {fileID: 100000, guid: 0cd47707143f91346868bfc349e4c4e0, type: 3}
+    Prefab: {fileID: 6473031571298860035, guid: 9f538e60b8b98634b8952310b91dfba0, type: 3}
     SourcePrefabToOverride: {fileID: 0}
     SourceHashToOverride: 0
     OverridingTargetPrefab: {fileID: 0}

Assets/DefaultNetworkPrefabs.asset.meta

@@ -1,5 +1,5 @@
 fileFormatVersion: 2
-guid: 1124711eebe5b22409c64043c7d96691
+guid: 456770a930389f64aa321439ffc4bdb2
 NativeFormatImporter:
   externalObjects: {}
   mainObjectFileID: 11400000

Assets/External/Animations.meta

@@ -1,5 +1,5 @@
 fileFormatVersion: 2
-guid: a2f0cf2c628f1264b819dc553d3d4829
+guid: 149941bc8bfaf104b8ed0d1977818519
 folderAsset: yes
 DefaultImporter:
   externalObjects: {}