docs: guide native Apple setup through Clerk CLI skills

[seanperez29]

What does this solve? What changed?

• Route existing native iOS and macOS Xcode projects through native-aware clerk init and clerk doctor when the installed CLI exposes that support; retain a source-guided fallback for older CLI builds.
• Remove the up-front publishable-key and UI-style gates from basic Swift setup. Keep AuthView scaffolding and native Sign in with Apple explicit opt-ins, and preserve custom key loaders.
• Clarify agent-mode preview, consent, partial-state, Doctor, and Xcode build guidance across clerk-swift, clerk-setup, clerk-cli, and the router.
• Add native setup evaluation cases and align the skills README and Claude mobile-plugin description.

Deadline

No rush. Keep this draft and do not merge until native CLI support has shipped. This guidance is capability-gated so older installs continue to follow the manual path.

Other resources

Native CLI stack: clerk/cli#490, clerk/cli#455, clerk/cli#486, clerk/cli#488.

Validation: frontmatter and JSON parsing, git diff --check, repository docs-link tests, full docs-link check, and two read-only skill forward evaluations.