This main file mostly covers using 2nd identity on an existing machine. The new machine directory covers new machine setups.

To remove old mis-committed branch:

git switch --orphan new-main

Alternative ssh key from main:

git config core.sshCommand "ssh -o IdentitiesOnly=yes -i ~/.ssh/HMRWork42_rsa -F none"

Set up redirection temporarily for initial clones:

~/.ssh/config

Host HMRWork42-github
  HostName github.com
  AddKeysToAgent yes
  IdentityFile ~/.ssh/HMRWork42-rsa

This allow clones with git clone git@HMRWork42-github:HMRWork42/HelloWorld, but not pushes. Pushes requires the git config line about, and also editing the url line in .git/config to revert back to standard non-redirected URL.

Found later, it is much easier to just set GIT_SSH_COMMAND to the same value as core.sshCommand during the initial clone!

Settings -> Emails -> Keep my email addresses private and Block command line pushes that expose my email