Document provider add --auth oauth-device for OpenAI CLI setup

[Aaronontheweb]

The model CLI commands are documented in src/content/docs/cli/model.md:

• netclaw model set [--context-window ]
• netclaw model discover

However, the provider CLI docs are inconsistent for OpenAI OAuth:

• src/content/docs/cli/provider.md says: "OpenAI OAuth is only available through this TUI flow".
• The same page's provider add usage omits --auth.
• src/content/docs/configuration/managed-providers.md documents: netclaw provider add my-openai openai --auth oauth-device.
• Current CLI help also supports: netclaw provider add openai --auth oauth-device.

Please update /cli/provider/ to document --auth , include oauth-device in provider add usage, and remove or revise the TUI-only OpenAI OAuth statement.