The image-assembly guard (build/mother-ssh-key, #113) refuses to copy the
mother key into a release image, but only after a full build run. Add the
same check to check_release_gate so a BUILD_CHANNEL=release build with the
key present on the host aborts in seconds, not after fetch/build/assemble.
The assembly-time guard stays as defense in depth.
(BUILD_CHANNEL already defaults to dev in build.cfg:17, so no change needed
there.)
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
