lith/mirror: add github_mirror.pub

Public half of the github_mirror keypair (private already tracked
encrypted as github_mirror.gpg). Lets the mirror systemd unit's
authorized_keys + GitHub deploy-key setup be reproduced from the
repo without round-tripping the vault.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>