Add common/doc/CONTRIBUTING.md

[cgwalters]

This came up in a live chat. I would like to have an at least somewhat clear policy for AI contributions. We should probably not try to invent something from scratch here.

I'm going to do some research on this, but in a nutshell the way I'm leaning is something like this:

• AI is definitely allowed
• If contributions are nontrivial, please add Assisted-by: <tool+model> into the commit message and PR description, like Assisted-by: Claude Code or Assisted-by: goose+gemini-2.5-pro
• AI is safer for generating unit tests and basic docs, but especially if AI provided substantial changes to "core logic" we ask that you make that explicit

[cgwalters]

One half of this will be done partially by #30 but another important half is to get a common blurb for humans into a common CONTRIBUTING.md etc, but that's a slightly bigger step.