When the working copy of your git repo resides on a remote box R1 rather than your local machine. You want to ssh into the R1, edit files and commits there and

When the working copy of your git repo resides on a remote box R1 rather than your local machine. You want to ssh into the R1, edit files and commits there and then push from there to some other remote host R2. Meanwhile your private key for accessing R2 is on your local machine. R1 can only access your forwarded agent, but not your private key. ProxyJump doesn't help you here because R1 does much more than just proxy your SSH connection.