Support all slash commands in ACP

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

Copilot CLI doesn't advertise slash commands via ACP

### Proposed solution

Just advertise **all** slash commands via the ACP protocol.

### Example prompts or workflows

_No response_

### Additional context

This was brought up in https://github.com/github/copilot-cli/issues/1044 , it's been closed but the last comment hints at something important:

> The core issue is about consistent experience across surfaces. In the terminal, slash commands aren't just shortcuts — they shape how I think about and drive the prompt.

I am opening this as a feature request rather than a bug. Without advertising slash commands, the Copilot CLI is not really implementing ACP.

Please keep in mind the following when suggesting other mechanisms to achieve similar outcomes to _some_ slash commands:

1. The suggestions in the bug linked above addressed some commands, not all. Having some commands available via ACP but not others makes the transition between CLI and ACP more difficult.
2. Most (all?) other CLI assistants implement this properly. There's value for Copilot CLI to have feature parity - it's not in this project's best interest to do things different for the sake of it.

Thank you!