How to fix SSH 'Too many authentication failures' — IdentitiesOnly

Your agent holds many keys and the server rejects you before SSH ever tries the right one.

Every flag and config keyword was checked against the OpenSSH 9.6 man pages, and the config examples were parsed for real with ssh -G.

Host web
    HostName example.com
    User deploy
    IdentityFile ~/.ssh/id_ed25519
    IdentitiesOnly yes