ci: gate develop & release image builds on the test suite

The Docker-image builds ran independently of the Test workflow, so a
failing test would not block publishing the :develop image (or a
release). GitHub Actions `needs:` only works within one workflow, so the
two separate workflows didn't depend on each other.

Make test.yml a reusable workflow (workflow_call) and call it from
develop.yml and release.yml as a `test` job that `build` depends on
(`needs: test`); release's `release` job already needs `build`, so it
waits transitively. test.yml keeps its pull_request trigger for PR
gating; its redundant push:develop trigger is dropped (develop.yml now
calls it on push).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/release.yml

@@ -19,7 +19,12 @@ env:
   IMAGE: ghcr.io/vvzvlad/gitmost
 
 jobs:
+  # Run the reusable test suite first so a failing test blocks the image build.
+  test:
+    uses: ./.github/workflows/test.yml
+
   build:
+    needs: test
     strategy:
       matrix:
         include: