Upgrade Astryx to 0.5.0

[walishcs]

Summary

• upgrade Astryx core and CLI from 0.4.4 to 0.5.0
• upgrade Stone, Neutral, Chocolate, and Matcha themes to 0.5.0
• refresh the Astryx-managed AGENTS.md block
• retain StyleX 0.19.0 and React 19 compatibility

Migration impact

• official 0.5.0 codemod scanned 38 source files and required no source changes
• no component API, hydration, theme, search, or blog outline regressions found

Validation

• npm ci
• npm run check
• npm test
• npm run format:check
• npm run build
• npm run astryx -- doctor
• browser smoke test: homepage, theme switch, search palette, blog Outline IDs, and runtime console