[IchenDEV]

Summary

This PR ships the current SkillsUI worktree as one draft change set.

• switch SKILL.md rendering from swift-markdown text output to a native Textual renderer with custom heading, quote, and code block styling
• stabilize sidebar/detail selection by tracking selected skill IDs and cache markdown render state off the parsed content hash
• add a distributable macOS packaging flow that builds SkillsUI.app and SkillsUI.dmg, including a bundled app icon, Info.plist template, ad-hoc signing, and drag-to-Applications DMG layout
• refresh the English and Chinese README docs and add repo-local agent guidance in AGENTS.md

Why

The app previously built as a plain SwiftPM executable, so it did not have a normal macOS install flow or a bundled Finder/Dock icon. At the same time, the in-app markdown preview was still using a limited attributed-string path, which made richer SKILL.md content harder to read and style.

Impact

• developers can now run ./scripts/package-dmg.sh to produce dist/SkillsUI.app and dist/SkillsUI.dmg
• packaged builds now include a sharper bundled app icon and standard app metadata
• markdown-heavy skill detail pages render with more native block styling and more stable detail refresh behavior

Validation

• DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer ./scripts/package-dmg.sh
• launched the packaged app from dist/SkillsUI.app
• copied the packaged app into /Applications as a temporary smoke test and confirmed it launched there as well
• mounted the generated DMG and verified it contained SkillsUI.app and the /Applications drag target

Notes

• the packaging flow is still local-distribution oriented: ad-hoc signed by default, not Developer ID signed, and not notarized
• this PR intentionally includes the full current worktree, including repo metadata updates, because the request was to commit and publish everything currently staged in the checkout