> Do you know if this already works with GitHub? It does not. From an `ssh -vvv ...

> Do you know if this already works with GitHub?

It does not. From an `ssh -vvv git@github.com -i <path/to/sk key>` session:

    debug1: Next authentication method: publickey
    debug1: Offering public key: <path redacted> ED25519-SK SHA256:<pubkey redacted> explicit authenticator
    debug3: send packet: type 50
    debug2: we sent a publickey packet, wait for reply
    debug3: receive packet: type 51
    debug1: Authentications that can continue: publickey

Packet type 51 is SSH_MSG_USERAUTH_FAILURE (see https://www.ietf.org/rfc/rfc4250.html#section-4.1.2).