ci: add checkstyle reporting, caching, checksums to workflows

.github/workflows/build.yml

@@ -6,6 +6,10 @@ on:
   pull_request:
     branches: [main]
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   build:
     name: Build LoChat
@@ -20,6 +24,7 @@ jobs:
         with:
           java-version: '21'
           distribution: 'temurin'
+          cache: 'gradle'
 
       - name: Setup Gradle
         uses: gradle/actions/setup-gradle@v4
@@ -27,6 +32,13 @@ jobs:
       - name: Build & Test
         run: chmod +x gradlew && ./gradlew build --no-daemon
 
+      - name: Checkstyle Report
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: checkstyle-report
+          path: build/reports/checkstyle/
+
       - name: JaCoCo Report
         if: always()
         uses: actions/upload-artifact@v4