The Copilot CLI should offer an option to deselect a custom agent

[doggy8088]

Describe the feature or problem you'd like to solve

Copilot CLI can select a custom agent but unable to deselect

Proposed solution

When I enter /agent in the TUI, I can select custom agent.

When I want to deselect the agent, I just can't right now. There is no way to deselect custom agent. I think it should have a way to deselect custom agent.

My use scenario is as follows:

1. I first switch to the Plan agent.
2. Then I want to start development, so I want to return to the default state where no Custom Agent is selected.

Example prompts or workflows

N/A

Additional context

N/A