Add doc/contributing/ai-contributions.md banning LLM-generated content from commits. Scoped to committed content only, excluding discussion, vendored deps, and accessibility tools. Enforcement uses consensus seeking. Rationale covers reviewer burden and DCO compatibility.

References added in CONTRIBUTING.md and the collaborator guide.

A PR was opened with a more relaxed AI policy (#62105), so this one here is offered as an alternative. Note, however, that some parts of that PR seem compatible with this one (e.g. the behavioural guidelines).

Some key points:

• I tried to draw a lot from existing AI bans in other projects. There's no need to tread entirely new ground here.
  • This list is slightly out of date, so in addition to these, I also had a look at Wikipedia's ban.
• In the recent discussions around AI usage in Node.js core, I found that some conversations (backchannel and otherwise) centered around whether an AI ban would be enforceable. I think consequences of policy violation can definitely be enforceable, but detection of policy violations is a lot more fuzzy, and that's okay. I tried to capture that here. A useful comparison is to local laws and policing. Not all law violations are universally detectable, but the ability to enforce them still ensures they're a deterrent.
• I've made an attempt at making reasonable exceptions. Other reasonable exceptions could be added.

This goes without saying, but I'm going to say it anyway, since this subject is pretty heated: This is not an invitation for personal attacks on myself, the author of the other PR, anyone who wants to ban AI in Node.js core, or anyone who doesn't want to ban AI in Node.js core. Or anyone else for that matter. Please observe Node.js' Code of Conduct.

EDIT: Just to be absolutely clear, this PR was made on my own, and has nothing to do with any of my employers, past or present.