ci: add verified plugin build workflow

Build and test with JDK 17 while preserving the Java 11 bytecode target, cache Gradle dependencies, and upload the packaged Mirai plugin.

.github/workflows/build.yml

@@ -0,0 +1,47 @@
+name: Build and Test
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: build-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v7
+        with:
+          persist-credentials: false
+
+      - name: Set up JDK 17
+        uses: actions/setup-java@v5
+        with:
+          java-version: '17'
+          distribution: temurin
+
+      - name: Set up Gradle
+        uses: gradle/actions/setup-gradle@v6
+
+      - name: Build and test
+        run: |
+          chmod +x gradlew
+          ./gradlew build buildPlugin --console=plain
+
+      - name: Upload plugin
+        uses: actions/upload-artifact@v7
+        with:
+          name: JChatGPT-${{ github.sha }}
+          path: build/mirai/*.mirai2.jar
+          if-no-files-found: error
+          retention-days: 14