revenuecat-migrate: migrate to RevenueCat or upgrade the SDK

Use this skill when the user wants to either adopt RevenueCat in an app that already ships in app purchases, or upgrade the RevenueCat SDK across a major version.

These two paths share some concepts but have different risks. Identify which one applies before touching code.

1. Detect the platform

Inspect the working directory and pick the first match, from top to bottom:

1. React Native: package.json has a react-native-purchases entry, or react-native as a dependency → read platforms/react-native.md. If expo is also a dependency, note it as an Expo project.
2. Flutter: pubspec.yaml exists at the project root → read platforms/flutter.md.
3. Kotlin Multiplatform: build.gradle.kts contains a kotlin { … } multiplatform source sets block, or depends on com.revenuecat.purchases:purchases-kmp* → read platforms/kmp.md.
4. Android (native): build.gradle(.kts) applies com.android.application (and is not KMP) → read platforms/android.md.
5. iOS (native): Package.swift, *.xcodeproj, *.xcworkspace, or Podfile at the project root → read platforms/ios.md.

If several match (e.g. an ios/ folder inside a Flutter project), pick the outermost project, the one that owns the build. If still ambiguous, ask the user which platform they want to configure.

2. Identify the migration path