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.
Yeah, WWW is really just text but that doesn't mean you don't need HTTP + HTML and a browser/search engine. Skills is just that, but for agent capabilities.
Long term you're right though, agents will fetch this all themselves. And at some point they will not be our agents at all.