@prologic@twtxt.net aha, thank you, that got me unjammed.
Turns out I thought I had an SSH key set up in github, but github didn’t agree with me. So, I re-added the key.
I also had to modify the command slightly to:
ssh -p 2222 -i PRIVATE_GITHUB_KEY GITHUB_USERNAME@cas.run help
since I generate app-specific keypairs and need to specify that for ssh and I haven’t configured it to magically choose the key so I have to specify it in the command line.
Anyhow, that did it. Thanks!