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.
I wish they arranged it around READMEs. I have a directory with my tasks and I have a README.md there - before codex had skills it already understood that it needs to read the readme when it was dealing with tasks. The skills system is less directory dependent so is a bit more universal - but I am not sure if this is really needed.
This is different from swagger / OpenAPI how?
I get cross trained web front-end devs set a new low bar for professional amnesia and not-invented-here-ism, but maybe we could not do that yet another time?
Because the descriptions aren't API specs and the things described aren't APIs.
Its more like a structure for human-readable descriptions in an annotated table of contents for a recipe book than it is like OpenAPI.