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
If you've tried ssh-add and you're still prompted to enter your passphrase then try using ssh-add -K. This adds your passphrase to your keychain.
Update: if you're using macOS Sierra then you likely need to do another step as the above might no longer work. Add the following to your ~/.ssh/config:
Host *
UseKeychain yes