[Enhancement] Add --uninstall command to remove tools

```
Add a --uninstall flag that presents the same checkbox UI 
but runs the removal command for each selected tool:

- macOS:   brew uninstall <tool>
- Linux:   sudo apt-get remove <tool>
- Windows: winget uninstall <tool>

This makes dev-setup a complete environment management tool, 
not just an installer.
```