Move the GHCR mirror workflow to go-mantis

The mirror workflow needs to live on a default branch to be scheduled/dispatched,
and go-mantis (default: main) is the repo that consumes these images. Keep this
branch scoped to the compose change that points azurite/azure-cli at the mirror.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: markt-sublime