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>

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