ci: Add GitHub Actions workflow for build and test

.github/workflows/ci.yml

@@ -0,0 +1,40 @@
+name: CI
+
+on:
+  pull_request:
+    branches: [ main, dev ]
+  push:
+    branches: [ main, dev ]
+
+concurrency:
+  group: ci-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  test:
+    name: Build & Test
+    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'
+
+      - name: Set up Gradle
+        uses: gradle/actions/setup-gradle@v4
+
+      - name: Build & Test (ArchUnit 계층 규칙 포함)
+        run: ./gradlew build --no-daemon
+
+      - name: Upload test report
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: test-report
+          path: build/reports/tests/test
+          if-no-files-found: ignore
+          retention-days: 7