meta: add a root-level AGENTS.md

We're seeing a number of repeated mistakes in AI-generated
contributions, including agents using `Signed-off-by`, creating
what appear to be unsupervised PRs, appearing to respond to
comments without human oversight, etc. Having an AGENTS.md
file with specific instructions to the agent on what to avoid
should help.

Signed-off-by: James M Snell <jasnell@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/65493
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Filip Skokan <panva.ip@gmail.com>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Chemi Atlow <chemi@atlow.co.il>
Reviewed-By: Daeyeon Jeong <daeyeon.dev@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Mike McCready <66998419+MikeMcC399@users.noreply.github.com>
Reviewed-By: Beth Griggs <bethanyngriggs@gmail.com>