Yubikey as an SSH key
All other guides I've seen (https://github.com/drduh/YubiKey-Guide being the most prolific) tell you to use the Yubikey's smartcard (PKCS#11) features with GnuPG via gpg-agent.
STOP THE MADNESS!
OpenSSH has supported OpenSC since version 5.4. This means that all you need to do is install the OpenSC library and tell SSH to use that library as your identity.