Support Git Bash

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

Support switching CoPilot to use Git Bash instead of Powershell on Windows.

### Proposed solution

I use the Claude CLI at work but have a CoPilot subscription for a side project so thought I'd give the CoPilot CLI a try. Well, much to my surprise, it was forcing me to install Powershell 7 when I have a perfectly good, working bash implementation (Git Bash via Git For Windows) that neither Claude CLI nor CoPilot for VSCode have any issues with. I tried for over 30 minutes to get it to use Bash for terminal commands to no avail. It seems like its a hardwired dependency. Any chance this is on the roadmap?

### Example prompts or workflows

> Use Git Bash commands for any terminal commands you run instead of Powershell.

### Additional context

_No response_