◆結論
https://stackoverflow.com/questions/21527743/mysql-workbench-version-6-0-8-ssh-authentication-issue
↑で
You need to ensure that your private key is in openssh format. With puttygen you can export as Openssh. This worked for me.
とかいうコメントがあって「は？何言ってんだコイツ」と思いつつ試したら動いた。
うおおお死ね。