IME, multiple (good) AGENTS.md is even better. I mostly see them only at the root of a repository, but I spread more out into important subdirectories. They act as a table of contents and spark notes. Putting more focussed AGENTS.md in important places has been even more helpful.

Bonus points if you can force them into context without needing the agent to make a tool call, based on touching the files or systems near them. (my homegrown agent has this feature)