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_

先追蹤 /diff 指令和 copilot 的工作流程。確認工作進行中 /diff 的行為，然後讓使用者可以在不中斷主要 agent 工作流程的情況下檢視即時變更；完成的標準是可以在同一個終端機中檢視長時間執行的工作。

由索引模型根據 Issue 內容生成。