Add iOS App Intents skill to iOS plugin

[dimillian-oai]

Summary

• add a new ios-app-intents skill to the build-ios-apps plugin
• document the new skill in the plugin README
• update plugin metadata and prompts so App Intents work is discoverable

Why

• expose App Intents guidance alongside the plugin's existing SwiftUI and Xcode workflows
• keep the marketplace-facing metadata aligned with the plugin's actual capabilities

Validation

• parsed plugins/build-ios-apps/.codex-plugin/plugin.json with python3 json.load
• manually reviewed the plugin diff, including the new skill reference docs
• repo-local scripts/validate_plugin.py was not present in this checkout, so that validator could not be run here