- GetFile uses new GetFileResponse instead of manual FileResponse construction
- ListFiles handler parses optional user_id query parameter
- Wire FolderStore into FileService in app.go, testenv, and file_test
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>