feat: add shared agent skills index and integrate SwiftUI skills for Antigravity compatibility

.agents/skills/index.md

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+# Shared Agent Skills Index
+
+Skills available through the shared `.agents/` surface for Antigravity-compatible workflow views and other tools that read the Agent Skills open format.
+
+---
+
+## Generic Skills
+
+- [Workspace Memory Curation](workspace-memory-curation/SKILL.md)
+- [Professional Communication](professional-communication/SKILL.md)
+- [Status Reporting](status-reporting/SKILL.md)
+- [AI Prompt Engineering](ai-prompt-engineering/SKILL.md)
+
+---
+
+## Fidelity / iOS Skills
+
+- [Copilot Prompt Engineering](copilot-prompt-engineering/SKILL.md)
+- [iOS Swift Answering](ios-swift-answering/SKILL.md)
+- [iOS Testing Strategy](ios-testing-strategy/SKILL.md)
+- [SwiftUI Expert Skill](swiftui-expert-skill/SKILL.md)
+- [SwiftUI XFlow Review](swiftui-xflow-review/SKILL.md)
+- [Update SwiftUI APIs](update-swiftui-apis/SKILL.md)
+
+---
+
+## Compatibility Notes
+
+- `.agents/skills/` is the shared skill source intended for Antigravity-compatible and cross-tool discovery.
+- `.opencode/skills/` mirrors the installed skills for OpenCode runtime discovery.

.agents/skills/swiftui-expert-skill

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+../../vendor/agent-skills/SwiftUI-Agent-Skill/swiftui-expert-skill

.agents/skills/update-swiftui-apis

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+../../vendor/agent-skills/SwiftUI-Agent-Skill/.agents/skills/update-swiftui-apis