feat: Allow configuring a separate model for subagents

Hello,

I would like to request an option to change the model used by subagents, since they currently always pick up the same model as the main chat.

It would be useful to be able to delegate subagent tasks to a weaker, faster model, while keeping the more capable model for the main conversation.

Acceptance criteria:

• A new setting under Sub-agents allows selecting the model used by sub-agents.
• When a sub-agent model is configured, sub-agents use that model instead of the main chat's model.
• When no sub-agent model is configured, the current behavior is preserved (sub-agents use the main chat's model) as the default fallback.

Thank you for the great work on cptr!

Best regards,
Joshua Krimmer

Start by tracing the Sub-agents settings entry point and the code path that selects the model for subagent tasks; no specific files or tests are named in the issue. Add a separate optional subagent-model setting, use it when configured, and preserve the main chat model as the fallback. Verify the setting and both configured and default behaviors.