CHANGES
- Add -V/--vendor flag to specify model vendor
- Implement vendor-aware model resolution and availability validation
- Warn on ambiguous models; suggest --vendor to disambiguate
- Update bash, zsh, fish completions with vendor suggestions
- Extend --listmodels to print vendor|model when interactive
- Add VendorsModels.PrintWithVendor; sort vendors and models alphabetically
- Pass vendor through API; update server chat handler
- Standardize docs and errors to --yt-dlp-args="..." syntax
- Add test covering ambiguous model warning across multiple vendors
- Promote go-shellquote to direct dependency in go.mod
