ci: add GitHub Actions workflow to run the build on PRs and main

The quality gates (tests, ktlint, detekt, apiCheck, explicit-API strict mode,
allWarningsAsErrors, and the aggregate 80% Kover coverage floor) previously ran
only on developer machines, so a PR could merge while breaking any of them.

Add .github/workflows/ci.yml that runs `./gradlew build` on every pull request
and on pushes to main. `./gradlew build` already chains the full gate, so the
workflow stays minimal:

- Checks out submodules recursively (the styleguide directory is a submodule).
- Installs a single Temurin JDK 21. The foojay-resolver-convention plugin
  auto-provisions the JDK 8 and JDK 11 toolchains the modules need, and JDK 21
  covers the virtual-threads module. JDK 21 (not the newest LTS) is deliberate:
  detekt 1.23.x crashes on JDK 25+, so the daemon stays on 21 to keep detekt green.
- Uses gradle/actions/setup-gradle for dependency and build caching.

Also add a CI status badge to the README.

Author: OmarAlJarrah

.github/workflows/ci.yml

@@ -0,0 +1,46 @@
+name: CI
+
+# Run the full Gradle build on every pull request and on pushes to main.
+# `./gradlew build` runs the complete quality gate: tests, ktlint, detekt,
+# apiCheck (binary-compatibility), explicit-API strict mode, allWarningsAsErrors,
+# and the aggregate 80% Kover line-coverage floor.
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+# Cancel superseded runs on the same ref so only the latest commit of a branch/PR builds.
+concurrency:
+  group: ci-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v4
+        with:
+          # The styleguide directory is a git submodule; fetch it so the working tree matches local checkouts.
+          submodules: recursive
+
+      # Install a single Temurin JDK 21. The build's Gradle daemon runs on this JVM, and the
+      # foojay-resolver-convention plugin (settings.gradle.kts) auto-provisions the JDK 8 and
+      # JDK 11 toolchains the Java-8 and JDK-11 modules require. JDK 21 also satisfies the
+      # virtual-threads module's toolchain. JDK 21 (rather than the newest LTS) is deliberate:
+      # detekt 1.23.x crashes on JDK 25+, so keeping the daemon on 21 keeps the detekt gate green.
+      - name: Set up JDK 21
+        uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: '21'
+
+      # Built-in Gradle dependency and build caching, plus wrapper validation.
+      - name: Set up Gradle
+        uses: gradle/actions/setup-gradle@v4
+
+      - name: Build
+        run: ./gradlew build

README.md

@@ -7,6 +7,7 @@
 
 <h1 align="center">Java SDKs Platform</h1>
 
+[![CI](https://github.com/dexpace/java-sdk/actions/workflows/ci.yml/badge.svg)](https://github.com/dexpace/java-sdk/actions/workflows/ci.yml)
 [![License](https://img.shields.io/badge/license-MIT-blue.svg)](LICENSE)
 [![Kotlin](https://img.shields.io/badge/kotlin-2.3.21-7F52FF.svg?logo=kotlin&logoColor=white)](https://kotlinlang.org)
 ![JDK](https://img.shields.io/badge/JDK-8%2B-437291.svg?logo=openjdk&logoColor=white)