Refine README showcases and structure

[Mine77]

What changed

• round the purple StarryKit wordmark directly in the SVG
• replace the dense overview with three concise emoji-led value points
• reduce installation to the verified npx skills command and Hosted MCP URL
• move playable GIF demos directly below Install while retaining HD MP4 links
• pair Usage prompts with their visual showcases
• add Metrics followed by a table-based Features section
• remove the How it works section

Why

GitHub README pages do not reliably render repository MP4 files as inline players. Optimized animated GIF previews make the product flow visible in place while the MP4 links preserve higher-quality playback.

Validation

• npm test (5/5 passing)
• npx -y skills add StarryKit/starrykit-plugin --list discovers starrykit-authoring
• both GIFs validated as multi-frame 768x432 animations
• git diff --check