ci: the eval image installs zsh, and the macOS lane sources the Codex probe from zsh

Dockerfile.ci adds zsh so the sourced-helper guard runs (and fails loudly
when zsh is missing) inside the eval container too. The macOS named
regressions job sources bin/gstack-codex-probe from zsh in an unrelated
directory and runs _gstack_codex_select_model exec (no paid call), then runs
the sourced-helper guard on the macOS runner. A free test pins both lanes and
runs the macOS step's script locally.

.github/docker/Dockerfile.ci

@@ -33,9 +33,11 @@ RUN printf 'Acquire::Retries "5";\nAcquire::http::Timeout "30";\nAcquire::https:
 #   bash bins shell out to it (macOS ships python3; the base image doesn't).
 # file: skill-validation's no-compiled-binaries-in-git check runs `file --mime-type`.
 # poppler-utils: make-pdf's e2e gates hard-require pdftotext/pdffonts/pdfinfo in CI.
+# zsh: the sourced-helper guard sources every skill helper under bash and zsh
+#   (macOS's default shell) and fails under CI=true when zsh is missing.
 RUN for i in 1 2 3; do \
       apt-get update && apt-get install -y --no-install-recommends \
-        git curl unzip xz-utils ca-certificates jq bc gpg python3 python3-venv file poppler-utils gcc libc6-dev && break || \
+        git curl unzip xz-utils ca-certificates jq bc gpg python3 python3-venv file poppler-utils gcc libc6-dev zsh && break || \
       (echo "apt retry $i/3 after failure"; sleep 10); \
     done \
     && rm -rf /var/lib/apt/lists/*