Add `/undo` slash comment in Copilot CLI

[doggy8088]

Describe the feature or problem you'd like to solve

Sometimes the Copilot CLI makes mistakes, and I want to revert changes, but there is no version control available.

Proposed solution

Implementing a /undo slash command for this scenario.

Example prompts or workflows

In VS Code, it's called "Restore Checkpoint".

Additional context

No response