Add SwiftUI and UIKit skills

[sreejithraman]

Summary

• add greenfield SwiftUI guidance for state, identity, navigation, and platform materials
• add greenfield UIKit guidance for controllers, scenes, traits, layout, and collection identity
• add a shared SwiftUI-first decision guide for choosing SwiftUI, UIKit, or a hybrid boundary
• fold current Liquid Glass guidance into SwiftUI and remove the standalone imported skill
• remove the retired skill’s source, credit, and unresolved license entries

Verification

• python3 scripts/check_skills.py
• git diff --check
• native review rounds completed; latest framework-choice review finding resolved
• Apple API availability checked against the installed Xcode 27 SDK headers
• framework-choice guidance checked against current primary Apple documentation and WWDC sessions

Notes

These are instructional skills, so there is no runtime app flow to exercise. The skills route visual and interactive product checks through manual-verify.