Add GitHub Actions workflow for automated testing

- Set up initial CI pipeline
- Configure .NET and Mono/F# environments
- Run test suite on Ubuntu
- Prepare for future cross-platform testing

.github/workflows/test.yml

@@ -0,0 +1,39 @@
+name: Tests
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+
+jobs:
+  test:
+    name: Test on ${{ matrix.os }}
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest]
+        # We can expand this later to include: macos-latest, windows-latest
+
+    steps:
+    - uses: actions/checkout@v4
+
+    - name: Setup .NET
+      uses: actions/setup-dotnet@v4
+      with:
+        dotnet-version: '8.0.x'
+
+    - name: Setup Mono/F#
+      if: matrix.os != 'windows-latest'  # Mono not needed on Windows
+      run: |
+        sudo apt-get update
+        sudo apt-get install -y fsharp
+
+    - name: Make scripts executable
+      if: matrix.os != 'windows-latest'
+      run: |
+        chmod +x fsi
+        chmod +x test_fsi.fsx
+
+    - name: Run tests
+      run: ./test_fsi.fsx