chore(ci): add aggregator job reporting the org-required 'ci / ci' context (#108)

proposal-system's seven CI jobs have distinct names, so the org ruleset's
required 'ci / ci' status check never reported here. The aggregator needs
all real CI jobs and fails if any failed or was cancelled. NOTE: this
check will be red until the pre-existing Python Lint/Tests failures
(INFRA-55) are fixed — it reports CI state honestly.

.github/workflows/ci.yaml

@@ -105,3 +105,16 @@ jobs:
       dotnet-version: "8.0.x"
       dotnet-publish-project: api/src/ProposalSystem.Api/ProposalSystem.Api.csproj
       run-typecheck: false
+
+  # Aggregator producing the org-required "ci / ci" status context.
+  # Caller/inline jobs here have distinct names, so the org ruleset's
+  # required check would otherwise never report on this repo.
+  ci:
+    name: ci / ci
+    needs: [dotnet, web, web-test, python, python-test, mobile, infra]
+    if: always()
+    runs-on: ubuntu-latest
+    steps:
+      - name: All CI jobs passed
+        run: |
+          [ "${{ contains(needs.*.result, 'failure') || contains(needs.*.result, 'cancelled') }}" = "false" ]