[ci skip] Add agent guidelines (#14172)

Not an official policy or stance, but should help steer agents and their users towards more thoughtful contributions. This is more of an experiment in reducing the triage workload without adding more strict processes like other projects have done (i.e. Ghostty, Pi, Herdr).

Things that might cause us to revisit this:
- Someone on the team starts using cloud agents for Paper or similar, we may need to adjust the maintainer check instruction
- The amount of tokens this adds (or behaviors it causes) ends up being a problem for us or external users who follow the guidelines
- Future models need less explicit instructions