I created keys as instructed in the github tutorial, registered them with github, and tried using ssh-agent explicitly — yet git continues to ask me for my passphrase every
It sounds like you may be having trouble with SSH-Agent itself. I would try troubleshooting that.
1) Did you do ssh-add to add your key to SSH?
2) Are you closing the terminal window between uses, because if you close the window you will have to enter the password again when you reopen it.