Run only script/cibuild in the Gitea workflow (closes #203)

The workflow's script/docker-smoke step is removed, so the workflow
checks out the repo and runs script/cibuild and nothing else, as the
canonical REPO_POLICIES.md says. The owner ruled on the issue that
make check covers what needs covering.

CI no longer starts the image and waits for its healthcheck.
script/docker-smoke and make docker-smoke stay, to run by hand, and
the script's header comment no longer says the workflow runs it. The
dated TODO.md entries that say the workflow runs it are history and
stay as written.

Model: opus-5-5

script/docker-smoke

@@ -1,8 +1,8 @@
 #!/bin/sh
 # script/docker-smoke: build the Docker image, start it, and wait up to
-# 30 seconds for its HEALTHCHECK to report healthy. It is not part of
+# 30 seconds for its HEALTHCHECK to report healthy. Needs a Docker
-# script/check, which never starts the image, and the Gitea workflow
+# daemon, so it is not part of script/check. Run it by hand; the Gitea
-# does not run it. Run it by hand.
+# workflow does not run it.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"