Document Xcode agent skill install flow

[AvdLee]

Summary

• Documents the Agentic Coding in Xcode skill install option for Xcode's built-in coding assistant.
• Clarifies that the RocketSim CLI operates on already-running Simulator apps and does not build, install, or launch from source.
• Updates CLI & Agent screenshots with the current settings UI.

Test plan

• npm run lint
• npm run format:check
• npm run typecheck
• npm run build
• npm run knip