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

script/test

@@ -11,9 +11,13 @@ ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
+    # The tag gets its own line: a failing command substitution inside
+    # an argument does not trip `set -e`, so the inline form degrades
+    # silently to an empty constant.
+    tag="$("$SCRIPT_DIR/projectname")"
     docker build --no-cache \
         --target test \
-        -t "$("$SCRIPT_DIR/projectname")-test" .
+        -t "$tag-test" .
 }
 
 main "$@"