Tag built images with the commit's short hash (closes #239)

Builds are tagged upaas-<app>:<short hash>, git's short form of
the commit checked out, instead of the deployment number. The clone
prints the short hash and fails without one.

A redeploy of a commit gives the tag to the new image. The cleanup
after a deploy now also finds the app's untagged images among those
its deployments recorded, and removes them by ID once the app neither
runs them nor would roll back to them. It keeps every image any app
uses, since apps that build the same commit can share one.

commit_sha is now saved on update so manual deploys keep the commit
read from the clone.

Model: opus-5-5

TODO.md

@@ -20,6 +20,15 @@ regress.
 
 # Completed Steps
 
+- 2026-10-01: Built images are tagged `upaas-<app>:<short hash>`, git's short
+  form of the commit built, instead of the deployment number. A redeploy of a
+  commit gives its tag to the new image; the old one is kept while the app runs
+  it or Rollback would start it, then removed by its ID, found among the images
+  the app's deployments recorded. The removal of old images now keeps every
+  image any app runs or would roll back to. A deployment's commit is now saved
+  when it is updated, so manual deploys keep the commit read from the clone
+  (#239).
+
 - 2026-10-01: Two database writes at the same moment no longer fail with
   "database is locked": each transaction now takes the write lock when it begins
   and waits up to 5 seconds for another writer to finish (#253).