Assign the image tag on its own line in script/ (closes #101)

script/cibuild, script/docker, script/lint and script/test passed the
tag from script/projectname inline to docker build, where a failing
command substitution does not trip set -e. Each now assigns it to `tag`
on its own line first, so the script stops where script/projectname
fails. The comment above it says why; the REPO_POLICIES.md snippets show
the same form.

Model: opus-5-5

TODO.md

@@ -21,6 +21,13 @@ fmt-check, and commit.
 
 # Completed Steps
 
+- 2026-10-05: `script/cibuild`, `script/docker`, `script/lint` and `script/test`
+  now assign the image tag from `script/projectname` on its own line before the
+  `docker build` (issue 101), so `set -e` stops the script where
+  `script/projectname` fails instead of running `docker build` with a broken
+  tag. The comment above it in each script says why, and the snippets in
+  `REPO_POLICIES.md` show the same form. Repositories pick this up on their next
+  re-vendor.
 - 2026-10-04: Went through the fleet findings recorded on 2026-08-09 (issue 62)
   and added the two rules `REPO_POLICIES.md` did not yet state: a new or changed
   check is proven by planting a defect it must catch; and a change to a separate