New setting for agents: Default Model

At the moment, you can assign a specific model to an agent, but doing so prevents you from selecting any other model.

It would be useful to be able to specify a default model for an agent and still allow the user to choose another if the situation requires it.

For example, I have xAi as my global default model. But when I choose my "Pro Coder" agent, I would like it to default to Claude Sonnet 4. However, there are times when using Pro Coder agent that I think GPT4.1 might be the better choice based on the type of coding question I'm asking.