AGENTS.md: add instructions for LLMs

AGENTS.md: add instructions for LLMs LLMs are pre-trained to look for a `AGENTS.md` when used in a coding harness. Nixpkgs has had an automation/AI policy but agents may or may not read it as they are not pre-trained to look for those files. `AGENTS.md` covers mostly all harnesses apart from Claude by Anthropic. For that we symlink Matrix thread :: https://matrix.to/#/!CTCrFzsBPYmDLmrja4:0upti.me/$VaREFlsVM_RniZfbTfyYyrPiuF_N7pTeWoBxVTD4pvI?via=nixos.org&via=matrix.org&via=tchncs.de --- `nixpkgs-review`, not sure if this worked correctly but it didn't throw any errors ```bash [reap@jager:~/Desktop/nixpkgs]$ nix-sh... While this is a weak guardrail, it is better than no guardrail to curb LLM slop. It seems to help other big projects like QEMU, Node.js, Rust, and systemd - https://github.com/qemu/qemu/blob/master/AGENTS.md Currently people may be inadvertently not following our documented process for code s... There is plenty of reasons why this hasn't been accepted in multiple past PRs, see https://github.com/NixOS/nixpkgs/pull/544374