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

`script/cibuild`, `script/docker`, `script/lint` and `script/test` passed the image tag from `script/projectname` inline in the `docker build` arguments. A failing command substitution inside an argument does not trip `set -e`, so the scripts went on to `docker build` with an empty, `-lint` or `-test` tag, against the rule their own comment states. Each now assigns `tag` on its own line before `docker build`, so the script stops where `script/projectname` fails. The snippets in `prompts/REPO_POLICIES.md` show the same form, and the policy states the own-line rule.

Consuming repositories pick this up on their next re-vendor; the defect failed closed.

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