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, and it used $version without saying where it comes from. The
item now gives the build as the canonical script/cibuild runs it, with
-t "$tag", and names the two steps the script runs before it, each on
its own line: the version from git describe --tags --always --dirty
(unknown if empty) and the tag from script/projectname. 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 +21,15 @@ fmt-check, and commit.
 
 # Completed Steps
 
+- 2026-10-07: The `script/cibuild` item in `NEW_REPO_CHECKLIST.md` now matches
+  the canonical `script/cibuild` (issue 125). Its image build carries the tag,
+  `-t "$tag"`, and the item says that `$version` comes from
+  `git describe --tags --always --dirty` (`unknown` if empty) and `$tag` from
+  `script/projectname`, each assigned on its own line before the build. A
+  repository written from the checklist got an untagged build, which leaves a
+  dangling image behind on every run, and was never told where `$version` comes
+  from. 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