https://docs.github.com/en/copilot/customizing-copilot/addin...
The first thing I do for a new project is ask Copilot to create a custom-instructions.md for me and then as I work on my projects, I ask it to update the instructions every now and then based on the current state of my project.
Much less misses this way in my experience.
I just spent last night working with Cursor and Claude Code, both support different styles of custom rules.
Then I got to work and was lamenting what my corp sponsored Copilot can't do the same - but turns out it can! :D
EDIT: Just tried it and Copilot is completely clueless. It can explain the file, but doesn't know how to generate one. Claude generates its own CLAUDE.md on startup. Cursor can create its own rulefiles pretty well, even better if you add a rule for generating rules.