Add AGENTS.md to guide AI-assisted contributions

[boranx]

Would we consider adding an AGENTS.md file to the repo?

AGENTS.md (https://agents.md/) is a small optional guide for AI coding agents. For conftest, it would simply note how to run the tests (go test ./..., bats acceptance.bats), source to conftest.dev for those that can fetch URLs, and point to the existing tests/ and examples/ directories for generating similar tests / scenarios OR nix ops to have the dev env up and running.