ci: add GitHub Actions workflow for build and test (#6)

Runs ./gradlew build on every push and pull request using Temurin
JDK 17, matching the version enforced in build.gradle.kts and
jitpack.yml. Uses gradle/actions/setup-gradle for dependency and
wrapper caching, and a concurrency group to cancel superseded runs
on the same ref.

https://claude.ai/code/session_01H1ckRKNGs7ffbVSUZua7zD

Co-authored-by: Claude <noreply@anthropic.com>

Author: brody-0125

.github/workflows/ci.yml

@@ -0,0 +1,33 @@
+name: CI
+
+on:
+  push:
+  pull_request:
+
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  build:
+    name: Build & Test (JDK 17)
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up JDK 17
+        uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: '17'
+
+      - name: Set up Gradle
+        uses: gradle/actions/setup-gradle@v4
+
+      - name: Make gradlew executable
+        run: chmod +x ./gradlew
+
+      - name: Build and test
+        run: ./gradlew build --no-daemon --stacktrace