feat: expose Brownfield Gradle Plugin as node module

[MrMuzyk]

Summary

• Include the Brownfield Android Gradle Plugin source in the published @callstack/react-native-brownfield package
• Add a script that copies the Gradle plugin source into the package before publishing and removes the copy afterwards
• Document the node_modules setup as an advanced option for local patching
• Keep the Maven dependency as the default and recommended setup
• Add missing repository declarations to the Gradle plugin's settings.gradle to support composite build resolution
• Bump Kotlin compiler to 2.2.21 for Gradle 9.x compatibility
• Fix VariantProcessor.kt nullability — mustRunAfter requires non-null under Kotlin 2.2
• Add a changeset

Test plan

Check that the npm package contains the Gradle plugin source:
yarn workspace @callstack/react-native-brownfield pack --dry-run

---

Check that the copied Gradle plugin project can be loaded by Gradle:

yarn workspace @callstack/react-native-brownfield sync:gradle-plugin
./gradle-plugins/react/gradlew -p packages/react-native-brownfield/gradle-plugin/brownfield tasks
yarn workspace @callstack/react-native-brownfield sync:gradle-plugin --clean

---

Within RNApp

1. Remove classpath("com.callstack.react:brownfield-gradle-plugin:") from android/build.gradle
2. Add to android/settings.gradle inside pluginManagement:

  includeBuild("../node_modules/@callstack/react-native-brownfield/gradle-plugin/brownfield")
  repositories {
      google()
      mavenCentral()  
      gradlePluginPortal()
  }

3. Run ./gradlew :BrownfieldLib:assembleDebug
4. Verify > Task :brownfield:compileKotlin appears in the output — this confirms the plugin was compiled from node_modules source rather than downloaded from Maven Central
5. Verify BUILD SUCCESSFUL

Within Expo55

1. Run expo prebuild --platform android --clean in apps/ExpoApp55
2. In the generated apps/ExpoApp55/android/settings.gradle, add inside the pluginManagement block (after the last includeBuild line, before the closing }):
   includeBuild("../node_modules/@callstack/react-native-brownfield/gradle-plugin/brownfield")
   repositories {
   google()
   mavenCentral()
   gradlePluginPortal()
   }
3. Run ./gradlew :brownfieldlib:assembleDebug
4. Verify > Task :brownfield:compileKotlin appears — confirms plugin compiled from node_modules source, not Maven
5. Verify BUILD SUCCESSFUL