Tag the image build in the checklist's script/cibuild item (closes #125)

The new-repository checklist gave script/cibuild's image build without
a tag, so a repository written from it left a dangling image behind on
every run. The item now gives the build as the canonical script/cibuild
runs it, with -t "$tag" and the tag assigned from script/projectname on
its own line. The other docker build commands the checklists and
REPO_POLICIES.md give for script/ already matched their scripts.

Model: opus-5-5

TODO.md

@@ -21,6 +21,13 @@ fmt-check, and commit.
 
 # Completed Steps
 
+- 2026-10-07: The `script/cibuild` item in `NEW_REPO_CHECKLIST.md` now gives the
+  image build with its tag, `-t "$tag"`, assigned from `script/projectname` on
+  its own line, as the canonical `script/cibuild` runs it (issue 125). A
+  repository written from the checklist got an untagged build, which leaves a
+  dangling image behind on every run. The other `docker build` commands the
+  checklists and `REPO_POLICIES.md` give for `script/` already matched their
+  scripts.
 - 2026-10-07: `script/lint` and `script/test` now build with
   `--output type=cacheonly` in place of a tag (issue 123), so they still run
   their phase uncached and fail on a failing step but write no image. Nothing