Allow setting default Agent #5383

Describe the feature or problem you'd like to solve

Allow configuring a default agent instead of requiring agent activation in each new session

Proposed solution

GitHub Copilot CLI already has configurable defaults for the following:

• Model
• Reasoning Level
• Context Size

My proposal is that a setting for default agent is introduced which should allow me to specify the agent name, and upon start of a session it would do the following:

1. Check the session for an existing selected agent

• This is to prevent overriding specifically selected agents in specific sessions

3. Attempt to load the configured default agent by name
4. If the agent cannot be loaded fall back to Default copilot agent

This would improve my workflow, and I'm sure the workflow of others who use one agent consistently as a baseline and want to not have to think about configuring it before firing off a request. I've had several times I started a new session after ending a long one and forgot to make the agent change, and had to start over.

Example prompts or workflows

For example, I have an agent called "Rover" and I want to default to using "Rover" with GPT-6 Luna at standard context and Max reasoning.

I would do /settings like I already do for setting the model defaults, and see an additional option for setting the default agent file.

Additional context

No response