[UX] Allow user to select the different model for different agent modes

[iambpn]

Describe your experience
I use copilot with plan mode and agent mode one after other and every time I switch between them, I find my self needing to switch the model since to implement the plan already generated by big model like opus small model is enough. Constantly needing to switch between the model along with agent is kind of frustrating, and sometimes I forget to change the model and go implement with the expensive model, tanking my premium request.

Suggestions for improvement
Allow users to keep selecting different models for different agent mode. For example if i select opus for plan model and gpt 5 mini for agent mode then when I switch back to plan mode from agent mode then it should select opus again instead of gpt-5-mini