revenuecat-purchase-flow: buy a package and restore purchases

Use this skill when the user wants to complete the purchase side of RevenueCat: fetch offerings, call purchase, deal with cancellation and errors, and expose a "Restore" action. It does not cover rendering a paywall UI (that lives in revenuecat-paywall) or gating features (that lives in revenuecat-entitlements-gate).

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. Shared concepts (all platforms)