it's actually by sourcegraph and it's been up since may.
Here's where it used to go: https://web.archive.org/web/20250702163859/ampcode.com/agent...
Here's their announcement
https://ampcode.com/news/AGENT.md
The openai thing is some recent partnership I'm guessing
Interesting enough, Sourcegraph had agent.md which now 301's to agents.md (with the s).