GHCR repos are now public and no longer require authentication by
default. Remove the baked-in XOR-encoded PAT and all automatic
credential injection. Image pulls are unauthenticated unless the user
explicitly provides credentials via --registry-username/--registry-token
flags or OPENSHELL_REGISTRY_USERNAME/OPENSHELL_REGISTRY_TOKEN env vars.

Also adds a new --registry-username CLI flag on gateway start for
private registry use cases, and removes the automatic community
registry credential injection (COMMUNITY_REGISTRY_* env vars) since
community sandbox images on GHCR are also public.