I am considering describing a SDE role for my agent to load in at context opening.

Not in one file, but in many.

I have seen people try to describe rules in an AGRNTS file but that is just too poor for my taste.

I wanted to describe rules for a lot things I have learned during my career, spanning from how to think about a feature to how much code can be in a PR, how a branding strategy should be, which tools and packages can be used and so on.

But in extended detail, and in a range of files not merely in one.

Someone else has done this I assume?