fix(docs): correct BYOK utility model default

[ShiroKSH]

Summary

• Correct the documented default utility model for BYOK main-agent models.
• Explain how to restore utility features when GitHub Copilot utility models are unavailable.

Root cause

The setting defaults to GitHub Copilot, but the note incorrectly said that no utility model is used by default.

Validation

• Checked the current VS Code setting registration and endpoint-resolution behavior.
• Ran whitespace checks and regenerated the documentation sidebars without extra changes.

[ntrogh]

Thanks @ShiroKSH