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

prompts/REPO_POLICIES.md

@@ -118,8 +118,9 @@ style conventions are in separate documents:
   and nothing else:
 
     ```sh
-    docker build --no-cache --target lint -t "$(script/projectname)-lint" .
-    docker build --no-cache --target test -t "$(script/projectname)-test" .
+    tag="$(script/projectname)"
+    docker build --no-cache --target lint -t "$tag-lint" .
+    docker build --no-cache --target test -t "$tag-test" .
     ```
 
     **A stage that is not the last one in the file is built only when the final
@@ -132,7 +133,9 @@ style conventions are in separate documents:
     **Every `docker build` in `script/` is tagged**, here and in
     `script/cibuild` and `script/docker`. An untagged build leaves a dangling
     image behind on every invocation, on every developer host and every CI
-    runner; a tagged one replaces the previous image.
+    runner; a tagged one replaces the previous image. Each script assigns the
+    tag on its own line before the build, so `set -e` stops it where
+    `script/projectname` fails.
 
     Inside a phase the tool is invoked directly — `golangci-lint`, `go test`,
     `eslint`, `prettier` — never through `make lint` or `script/test`, which are
@@ -434,13 +437,15 @@ style conventions are in separate documents:
   byte-identically across repos:
 
     ```sh
-    # Own line: a failing command substitution inside an argument does not
-    # trip `set -e`, so the inline form degrades to an empty constant.
+    # The version and the tag each get their own line: a failing command
+    # substitution inside an argument does not trip `set -e`, so the inline
+    # form degrades to an empty constant.
     version="$(git describe --tags --always --dirty 2>/dev/null || true)"
     [ -n "$version" ] || version="unknown"
+    tag="$(script/projectname)"
     docker build --no-cache \
         --build-arg VERSION="$version" \
-        -t "$(script/projectname)" .
+        -t "$tag" .
     ```
 
     `--always` makes an untagged repo yield an abbreviated commit hash rather