* refactor: migrate vc/minutes shortcuts to FileIO
- vc_notes: replace vfs.Stat + validate.SafeOutputPath + validate.AtomicWrite
with FileIO.Stat/Save for transcript download
- minutes_download: replace validate.SafeOutputPath + validate.AtomicWriteFromReader
with FileIO.Save, use FileIO.Stat for overwrite checks
- Use WrapSaveError to preserve original error messages