From AGENT.md to AGENTS.md

In May we added support for AGENT.md files to Amp. Not long after, OpenAI picked AGENTS.md (plural) for their agent and, instead of insisting on what we came up with, we decided that we'd prefer one standard.

Our condition: if OpenAI can get the agents.md domain, we'll switch. And that condition has now been met.

True to our word, and as of today, Amp will look for AGENTS.md files (while staying backwards compatible with existing AGENT.md files, of course)