Feature Request: Allow model selection for built-in agents (currently appears to use Sonnet)

Built-in agents currently seem to use Sonnet regardless of the selected model in the main session.

Since sub agents already support model: inherit, it would be great if:

1. Built-in agents could also use model: inherit to match the main session's model
2. Or add a setting to configure which model built-in agents should use

This would help maintain consistency when using models like Opus throughout the workflow.

<img width="518" height="279" alt="Image" src="https://github.com/user-attachments/assets/9c6728b9-0544-412b-8e09-b23291c9d7be" />