You'd put the things that aren't up to personal preference in there, like an AI disclosure / contribution policy or what changes should look like to be reviewable. You usually don't need to tell humans "don't change things for the sake of changing them" because we don't like to do more work than we have to, but agents do.

You can also include a compact "table of contents," describing what lives where in the project. It saves at most a little bit of exploration though so I don't know if it's really worth the maintenance

For agents (semi-autonomous) I think it's more similar to introductory developer docs. Which are usually committed.

Like formatters and git ignore configuration, some agent instructions are personal and those should not be commited. But some are worth sharing. Having formatter config in a repo is not uncommon. And in this case the file is only a link to the readme, so the agent reads the contrib guidelines.

A contributor still has global config for their personal preferences. It makes sense to me, anyway. My biggest issue is it's not deterministic. Someone else could have a version that works better or uses fewer tokens for the same thing.

At work we commit the whole .claude folder, though we carve out some space for personal agents and skills.