At the project level, there should be no need for an AGENTS.md. Whatever you might want to let agents know you would let a human know too, so you should be writing a README.md, CONTRIBUTING.md, ARCHITECTURE.md and whatever else.
Humans don't need it. Nor do they need to be reminded to launch subagents for simple tasks for example - which is another command that belongs in AGENTS.md.
For example, "don't write comments that reference things that you removed from the code" is commonly needed for coding agente and almost never needed for people.