My .md files are specific to my domain - game development using Unity. Something that is probably universal and is a good baseline for every project and domain:

- which libraries to use for standard problems - project structure, it helps if it has a name(vertical slice etc) - which things llm is free to edit, which not - naming, comments etc

And most importantly: "what not to do", and you should do this a lot: Whenever llm does something that is a code smell, make it fix it and make it add a new rule to agents.md.

"don't make any mistakes" /s