chore: MixamoAvatar 에셋을 External/Animations로 이동

- Assets/_Game/Animations/MixamoAvatar → Assets/External/Animations/MixamoAvatar

- ActorCore 에셋 External/Animations/ActorCore에 추가

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

Assets/_Game/Animations/MixamoAvatar.asset.meta

@@ -1,8 +0,0 @@
-fileFormatVersion: 2
-guid: b63434e5ef6b98d45865ec4417e31bd4
-NativeFormatImporter:
-  externalObjects: {}
-  mainObjectFileID: 9000000
-  userData: 
-  assetBundleName: 
-  assetBundleVariant: