You might as well just write instructions in English in any old format, as long as it's comprehensible. Exactly as you'd do for human readers! Nothing has really changed about what constitutes good documentation. (Edit to add: my parochialism is showing there, it doesn't have to be English)
Is any of this standardization really needed? Who does it benefit, except the people who enjoy writing specs and establishing standards like this? If it really is a productivity win, it ought to be possible to run a comparison study and prove it. Even then, it might not be worthwhile in the longer run.
This is true for MCP as well. You could just describe a bunch of command line tools in AGENTS.md and tell the LLM when and how to call them. It would simply take more effort to set up, at least for some tools.
This is where a comparison in productivity would return a meaningful result: how much does it make it easier to set up things like that.