Set up GitHub Actions CI workflow for builds and tests

.github/workflows/ci.yml

@@ -0,0 +1,45 @@
+name: CI
+
+on:
+  push:
+    branches: [ main, master ]
+  pull_request:
+    branches: [ main, master ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up JDK 21
+        uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: 21
+          cache: gradle
+
+      - name: Grant execute permission for gradlew
+        run: chmod +x gradlew
+
+      - name: Build and test
+        run: ./gradlew build --no-daemon --stacktrace
+
+  # Optional: run on Windows as well to catch OS-specific issues
+  build-windows:
+    runs-on: windows-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up JDK 21
+        uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: 21
+          cache: gradle
+
+      - name: Build and test
+        run: .\gradlew.bat build --no-daemon --stacktrace