Why not give the instructions on where to get 
    credentials in the original prompt?

We don't push code unless it's been QA'd, and the LLMs need to know the credentials every time they do automated QA. It's very rare for me have a session in that repo where they wouldn't need that info. So it's a great candidate for AGENTS.md

Another concrete example would be codebase conventions. We prefer lean models. Cross-model concerns go into service objects. Without this direction LLMs tend to default to stuffing too much code into the models themselves, as is the de facto standard for most MVC apps and therefore this is how LLMs are trained.

Even if they could discover our large codebase's conventions flawlessly on their own in every session, this absolutely would require nontrivial work repeated in every session: quite a few turns grepping, conversing with LSPs, or whatever.

So yes... we could describe our conventions in every single session by typing it right into the prompts... right after typing the directions to find the test credentials... and the other ten or twelve things we'd be telling it every single time...