ci: add build and upload jobs

.gitea/workflows/main.yaml

@@ -0,0 +1,31 @@
+name: Build and Upload JAR
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
+
+jobs:
+  build:
+    runs-on: [ubuntu-latest]
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up JDK 11
+        uses: actions/setup-java@v4
+        with:
+          distribution: 'temurin'
+          java-version: '21'
+
+      - name: Build with Gradle
+        run: chmod +x gradlew && ./gradlew clean build
+
+      - name: Upload JAR artifact
+        uses: christopherhx/gitea-upload-artifact@v4
+        with:
+          name: mineclass
+          path: build/libs/*.jar