After more trial and error, the answer seems to be
The Github custom domain should indeed be
I did not need the apex records. In fact, Github emailed me discouraging it. So I deleted the apex records.
- In OVH, I added a DNS CNAME entry like this:
The thing that was confusing me is that I thought that the CNAME entry needed to have a link to the original Github pages WITH the project name
jeti.github.io/matrix. That was wrong. The target is simply
jeti.github.io. (Note the period on the end).