Created codai/models/cache/__init__.py with:
- get_model_cache_dir()
- get_all_cache_dirs()
- get_cached_model_path()
- is_huggingface_model_id()
- download_huggingface_model()
- download_model()
- list_cached_models()
- remove_cached_model()
- remove_all_cached_models()

This extracts the cache-related functionality into a separate module.