I think Google has that.

Possibly these are becoming more common because of /r/overemployed.

Most companies don't want you working another W-2 job, but realize they can't just ban all consulting.

I have seen that in employment paperwork at a few companies. Generally, you just mention you have side jobs and they okay it. Or you ignore it entirely and nobody notices.

I don’t think so. Or at most it talks about “reasonable effort” or something vague like that.

/someone who discovered an over-employed person on his team and wondered the same thing

I think these might soon be called Soham clauses, to be a bit cheeky.

Employment contracts in the US are rare.