feat: add -V/--vendor flag and vendor-aware model selection

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

data/patterns/show_fabric_options_markmap/system.md

@@ -167,6 +167,8 @@ us the results in
                         Select the model to use. NOTE: Will not work if you
                         have set a default model. please use --clear to clear
                         persistence before using this flag
+  --vendor VENDOR, -V VENDOR
+                        Specify vendor for the selected model (e.g., -V "LM Studio" -m openai/gpt-oss-20b)
   --listmodels          List all available models
   --remoteOllamaServer REMOTEOLLAMASERVER
                         The URL of the remote ollamaserver to use. ONLY USE