ci: build, test, and run Signage inside a pinned Ekklesia on every change

Signage had no CI. Its tests passed while three defects shipped that
appear only inside Ekklesia, in a real browser. Every job now assembles
Signage into a pinned platform commit, as an instance does, and
installs that workspace with --frozen-lockfile:
- lint, typecheck, test: Signage's own checks;
- build: the platform's gates against exactly this module set (boundary
  rules, module manifest, migrations), then the build;
- integration: the deployment image from the platform's Dockerfile,
  then the platform's split-topology smoke. It pairs a kiosk display,
  restarts it on the same profile, and runs the scenarios in
  ci/display-scenarios.

The supporting files:
- ci/platform.env pins one platform commit. ci/assemble.sh fetches the
  platform's main and refuses a pinned commit that is not on it, so CI
  only runs reviewed platform code.
- ci/pnpm-lock.yaml is the assembled workspace's lockfile, regenerated
  with ci/update-lock.sh.

Same trust rules as the platform: actions pinned by SHA,
persist-credentials false, no secrets, no pull_request_target.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

ci/platform.env

@@ -0,0 +1,6 @@
+# The Ekklesia platform Signage's CI builds and tests against: one reviewed commit on the platform's
+# `main`, never a moving ref. ci/assemble.sh refuses a commit that is not on `main`.
+#
+# Move it deliberately: change the commit, run `bash ci/update-lock.sh`, and commit both.
+EKKLESIA_PLATFORM_REPOSITORY=https://git.riverofglorymn.org/River_of_Glory_Church/Ekklesia.git
+EKKLESIA_PLATFORM_COMMIT=f50e8f42018c15698709739e2ba54771e97df08a