Allow opening /diff while copilot is working

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

It'd be very useful to be able to view /diff while copilot is working on something without interrupting it

### Proposed solution

It'd be very useful to be able to view /diff while copilot is working on something without interrupting it. This would allow live reviewing changes without interrupting the main agent workflow. It would be especially useful for long running tasks spanning minutes or more to quickly assess changes without needing to switch over to a different terminal.

### Example prompts or workflows

_No response_

### Additional context

_No response_