The root Gemfile only ever declared the fastlane gem and lived next to fastlane configuration anyway. Moving it under fastlane/ matches that ownership and keeps the repo root cleaner.
release-gplay job now sets BUNDLE_GEMFILE=fastlane/Gemfile and runs ruby/setup-ruby with working-directory=fastlane so bundler-cache resolves the moved Gemfile. fastlane lanes still run from the repo root.
