I feel like we've missed a trick here. The purpose of AGENTS.md—to make metadata about a project available in a structured format—seems far more valuable than to limit it to just those projects that use AI or those projects that do, but don't require any other kind of automated processing.
Why not iterate on README.md with something like PARSEME.md?
> The purpose of AGENTS.md—to make metadata about a project available in a structured format
AGENTS.md is not "structured format", then it'd be JSON or YAML or some other fucked up format instead. Currently at least, it's fully freeform and you can put whatever there, agents will do there best to follow it.