Add rendered Markdown preview

[mikhutchinson]

Summary

• Adds SiriusMarkdown 0.5.4 as an Xcode package dependency.
• Adds an Edit / Preview mode switch for Markdown files in the skill detail pane.
• Renders preview content through MarkdownRenderSession and MarkdownDocumentView with a Skillz-matched document theme.
• Strips SKILL.md frontmatter from the rendered preview while leaving Edit mode unchanged.

Notes

• Edit mode and autosave behavior stay unchanged.
• Preview updates are debounced and prepared outside SwiftUI body evaluation.
• Screenshots were captured only under an isolated temporary home/defaults environment.

Validation

• xcodebuild -resolvePackageDependencies -project skillz/skillz.xcodeproj -scheme skillz
• xcodebuild -scheme skillz -destination 'platform=macOS' CODE_SIGNING_ALLOWED=NO build
• xcodebuild -scheme skillz -destination 'platform=macOS' -only-testing:skillzTests CODE_SIGNING_ALLOWED=NO test
• 49 unit tests passed