Support GitHub Copilot style instructions and skills

Hey, once again regarding to migration/using in tandem with GitHub Copilot.

I'm going to list several items for compatibility, if needed ill create a new issue for each, just let me know:

- Support custom dirs for skills discovery
- Specifically support .github/skills for project skill discovery, and ~/.copilot/skills for personal skill discovery.
- Support custom instruction files
- Discover .github/copilot-instructions.md by default like Claude.md
- Support globbing instruction files
- Support copilots path specific instruction format (https://docs.github.com/en/copilot/how-tos/copilot-cli/customize-copilot/add-custom-instructions#creating-path-specific-custom-instructions)
- glob by default GitHub's custom path instructions (.github/instructions/**/*.instructions.md)

I'm willing to try to implement any or all as needed