$ git push
ssh_exchange_identification: read: Connection reset by peer
fatal: Could not read from remote repository.
Please make sure you have the correct access rights
and the repository exists.
お？
ssh-agentに私有鍵が登録されてないとこういう切れかたするんだ。
$ ssh git@github.com
PTY allocation request failed on channel 0
Hi zunda! You've successfully authenticated, but GitHub does not provide shell access.
Connection to github.com closed.
これでパスフレーズを促されてpushできるようになりました。