Adds ios-scaffold.md reference that explicitly prohibits Package.swift +
.executableTarget for iOS apps (produces macOS CLI, not iOS app bundle),
requires project.yml + xcodegen generate to create a proper .xcodeproj,
and documents SwiftUI API availability tiers (iOS 16 vs 17). Adds iOS
anti-patterns 28-29 to universal-anti-patterns.md and wires the reference
into gsd-executor.md so executors see the guidance during iOS plan execution.
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>