enable user to set default model for Agent/Chat

[DevOps-zhuang]

• Copilot Chat Extension Version:
• VS Code Version:q.1.123.1
• OS Version:windows 11
• Feature (e.g. agent/edit/ask mode):all
• Selected model (e.g. GPT 4.1, Claude 3.7 Sonnet):all
• Logs:

Steps to Reproduce:

1. to save token, suggest each user can set the default model, like the Auto/GPT-5-mini. currently, the user can't set the defual model for all agent.
2. such setting can be override when the model is set in custome agent. like PM.md already set the claude 4.7 as default agent, it will overrie the user-wide default agent.
3. for the Utility model, why only 'Default' and 'Use the built-in default utility model' can be selected？ hope all models can be selected here. just like what it shows on agent.

[dmitrivMS]

@vritant24 Not exactly BYOK, but feels related to what you're working on?