Agents list print

### Describe the feature or problem you'd like to solve

List available agents in stdout

### Proposed solution

Right now, if I want to know the list of available agents in a specific CLI session I need to open it and call /agent.

What if I have some pre-step - like I do - in which I'd love to programmatically start a session choosing between available agents?

A `copilot --available-agents`outputting the list would solve the issue.

### Example prompts or workflows

_No response_

### Additional context

I'd love to implement this myself and propose a PR

Start by locating the CLI option-parsing entry point and the existing code behind the /agent command. Add a --available-agents path that prints the available agents to stdout without opening an interactive session, then verify the output in a CLI session or command test.

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