ci: add alls-green gate job for branch protection

Add a 'CI Success' job that depends on all other CI jobs (lint + test
matrix) and uses re-actors/alls-green to determine the overall result.
This single job can be set as a required status check in branch
protection rules instead of tracking each matrix entry individually.

Author: zurfjereluhmie

.github/workflows/ci.yml

@@ -62,3 +62,17 @@ jobs:
 
       - name: Test with coverage
         run: uv run pytest tests/ -q --cov=evalwire --cov-report=term-missing --cov-fail-under=85
+
+  # https://github.com/marketplace/actions/alls-green#why
+  ci-success: # This job does nothing and is only used for the branch protection
+    name: CI Success
+    if: always()
+    needs:
+      - lint
+      - test
+    runs-on: ubuntu-latest
+    steps:
+      - name: Decide whether the needed jobs succeeded or failed
+        uses: re-actors/alls-green@release/v1
+        with:
+          jobs: ${{ toJSON(needs) }}