Vendor Apple's swiftui-specialist agent skill for contributor use

[lemikegao]

Summary

• Vendor Apple's swiftui-specialist agent skill verbatim (Xcode 27.0 beta 2, 27A5209h, via xcrun agent skills export) under .agents/skills/ for contributor agents.
• Add a provenance README with license boundary and re-export procedure, a task-scoped AGENTS.md pointer for SwiftUI implementation/review work, a repo-convention agents/openai.yaml (explicit invocation only), and a .gitattributes whitespace exemption (Vendor/Sparkle precedent).
• Defer swiftui-whats-new-27 (SDK 27-only APIs; repo builds on Xcode 26.x, README marks it as the add-back when the toolchain moves) and the other five exported skills (not applicable here; modernize-tests conflicts with the XCTest ledger policy in docs/testing.md).

Validation

• diff -r against the raw export — byte-identical; only the repo-added agents/openai.yaml differs
• make guardrails — passed
• .agents/skills/rpce-contribution-check/scripts/preflight.sh commit — passed
• .agents/skills/rpce-contribution-check/scripts/preflight.sh push — passed

Notes

• One maintainer call: Apple publishes no redistribution license for these bundles, so the README marks the content as Apple-authored and outside the repo's Apache-2.0 grant (same posture as community mirrors). Flagging for explicit acceptance.
• Contributor-facing only; RP's AgentSkillCatalog also surfaces it as a slash skill on this workspace. Shipping skills to end users would be a separate product feature.
• Follow-up candidate: a repo-authored docs/architecture/swiftui-conventions.md (window/state ownership, AppKit interop, macOS-vs-iOS divergences Apple's skill doesn't cover).