ci: add alls-green action for single required check

Add a 'check' job that aggregates all CI job results using the
alls-green action. This enables configuring a single required
status check in GitHub branch protection settings.

Author: Kludex

.github/workflows/shared.yml

@@ -77,3 +77,12 @@ jobs:
 
       - name: Check README snippets are up to date
         run: uv run --frozen scripts/update_readme_snippets.py --check
+
+  check:
+    if: always()
+    needs: [pre-commit, test, readme-snippets]
+    runs-on: ubuntu-latest
+    steps:
+      - uses: re-actors/alls-green@05ac9388f0aebcb5727afa17fcccfecd6f8ec5fe # v1.2.2
+        with:
+          jobs: ${{ toJSON(needs) }}