Support more helpful GitHub Copilot instructions

[luncliff]

Targets Visual Studio Code environment

• https://github.com/luncliff/vcpkg-registry/blob/main/.github/copilot-instructions.md
• Custom instructions for portfile.cmake, *.patch, etc

References

• https://github.com/Code-and-Sorts/awesome-copilot-instructions
• https://github.com/dfinke/awesome-copilot-chatmodes
• https://github.blog/changelog/2025-07-23-github-copilot-coding-agent-now-supports-instructions-md-custom-instructions/