Support subfolders for skills to better organize them

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

Support subfolders for skills to better organize them

### Proposed solution

Got the below message when trying to move all my "testing" related skills into a subfolder under the skills folder. I have over 10 skills I've created and the flat folders are getting messy fast.

Support subfolders: C:\Users\\.copilot\skills\testing

"The Copilot CLI currently does not support subfolders under the skills\ directory.** It only scans the immediate children of ~/.copilot/skills/ for SKILL.md files. Nesting them in a testing\ subfolder breaks discovery. This would be a good feature request — being able to organize skills into categories. You could file that via /feedback in the CLI if you'd like."

### Example prompts or workflows

_No response_

### Additional context

_No response_