Remove the ancestor-check gate and the separate move-latest job.
On main pushes, the merge job now tags both :main and :latest in
a single imagetools create call. Releases still get :<tag> only.
Removed:
- move-latest job (ancestor check + retag dance)
- Decide whether to move :main step (ancestor check in merge)
- Compute tag step
- push_main gate on manifest push
- merge job outputs (nothing downstream needs them anymore)
825948edab