Add demo workflow for matrix jobs on single runner

- Single EC2 instance runs 3 matrix jobs sequentially
- Tests ubuntu-20.04, ubuntu-22.04, ubuntu-24.04
- Shows that matrix jobs run one at a time on the same runner
- 60 second grace period to allow all jobs to complete

Author: ryan-williams

.github/workflows/demo-matrix-single-runner.yml

@@ -0,0 +1,35 @@
+name: Demo – matrix on single runner
+on:
+  workflow_dispatch:
+  workflow_call:   # Tested by `demos.yml`
+permissions:
+  id-token: write  # Required for AWS OIDC authentication
+  contents: read   # Required for actions/checkout
+jobs:
+  ec2:
+    uses: ./.github/workflows/runner.yml
+    secrets: inherit
+  test-matrix:
+    needs: ec2
+    runs-on: ${{ needs.ec2.outputs.instance }}
+    name: Test on ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-20.04, ubuntu-22.04, ubuntu-24.04]
+    steps:
+      - name: Test on ${{ matrix.os }}
+        run: |
+          echo "Testing on ${{ matrix.os }}"
+          echo "Runner: $(hostname)"
+          echo "Time: $(date)"
+          echo "Simulating work..."
+          sleep 10
+          echo "Done!"
+      - name: Show system info
+        run: |
+          echo "=== System Information ==="
+          uname -a
+          echo "=== CPU Info ==="
+          lscpu | head -10
+          echo "=== Memory Info ==="
+          free -h

.github/workflows/demos.yml

@@ -48,9 +48,15 @@ jobs:
     uses: ./.github/workflows/demo-matrix.yml
     secrets: inherit
 
+  demo-matrix-single-runner:
+    needs: check-ci
+    if: ${{ needs.check-ci.outputs.should-run == 'true' }}
+    uses: ./.github/workflows/demo-matrix-single-runner.yml
+    secrets: inherit
+
   # Summary job to ensure all tests passed
   demos-passed:
-    needs: [demo-minimal, demo-multi-job, demo-full, demo-matrix]
+    needs: [demo-minimal, demo-multi-job, demo-full, demo-matrix, demo-matrix-single-runner]
     runs-on: ubuntu-latest
     steps:
       - run: echo "✅ All demos passed!"