Same. Everybody builds one, and then eventually realizes it makes the model stupid and confused. The right solution is pretty much always developer documentation in the repo, in roughly the same shape you'd make developer docs for humans that you want to be able to get up to speed about your codebase really quickly.

This has the benefit of being something you can easily read over every now and then to make sure the model hasn't baked in some random comment you made about a specific circumstance a month ago and made a weird religion out of it.