Add matrix job example workflow

Shows how to use unique labels (job-run_id-job-index) to
prevent matrix jobs from competing for the same runner.

Author: manascb1344

.github/workflows/matrix-example.yml

@@ -0,0 +1,43 @@
+# Matrix Job Example for Modal GitHub Runner
+#
+# This workflow demonstrates how to run matrix jobs on Modal runners.
+#
+# IMPORTANT: Each matrix job needs a unique label to get its own runner.
+# Without unique labels, multiple matrix jobs compete for the same runner
+# and get stuck in "queued" state.
+#
+# The pattern: job-${{ github.run_id }}-${{ strategy.job-index }}
+#   - github.run_id  = unique per workflow run
+#   - strategy.job-index = unique per matrix entry (0, 1, 2, ...)
+#   Combined, they guarantee 1:1 binding between job and runner.
+
+name: Matrix Example
+
+on:
+  workflow_dispatch:
+
+jobs:
+  test:
+    runs-on:
+      - self-hosted
+      - modal
+      - job-${{ github.run_id }}-${{ strategy.job-index }}
+    timeout-minutes: 15
+
+    strategy:
+      matrix:
+        python-version: ["3.10", "3.11", "3.12"]
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Python ${{ matrix.python-version }}
+        run: |
+          python${{ matrix.python-version }} --version || true
+          echo "Testing with Python ${{ matrix.python-version }}"
+
+      - name: Run tests
+        run: |
+          echo "Running tests on Python ${{ matrix.python-version }}"
+          echo "Runner OS: $(uname -s)"
+          echo "Runner arch: $(uname -m)"