Add GitHub Actions build workflow (#14)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jie65535 <29452349+jie65535@users.noreply.github.com>

.github/workflows/build.yml

@@ -0,0 +1,34 @@
+name: Build Plugin
+
+on:
+  push:
+    branches: [ master, main ]
+  pull_request:
+    branches: [ master, main ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up JDK 11
+        uses: actions/setup-java@v4
+        with:
+          java-version: '11'
+          distribution: 'temurin'
+
+      - name: Grant execute permission for gradlew
+        run: chmod +x gradlew
+
+      - name: Build Plugin
+        run: ./gradlew buildPlugin
+
+      - name: Upload Plugin Artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: plugin
+          path: build/mirai/