[AI] Add agent.md and copilot.md for this repo. #518
Description
Many open source projects have started adding AI guidance files to improve the developer experience when using LLM tools.
Eg: https://github.com/conda/rattler
This issues suggests adding:
- copilot.md – Custom instructions for GitHub Copilot to help it better understand the structure, conventions, and expected patterns in this repository.
- agent.md – Guidance for developers on how to effectively leverage LLMs while working with or using pyGAM.