docs: reframe README as WebMCP showcase

[ilayalog]

What

Reframes the README around WebMCP Computer as an enduring open-source showcase: one machine shared by a human and an AI agent. Adds a real product hero, a 24-second H.264 demo, a compact Mermaid architecture diagram, and a reproducible native-WebMCP recording driver.

Why

The previous README read like comprehensive challenge submission documentation. This version leads with the product, demonstrates dynamic WebMCP and WebMCP-inside-WebMCP visually, moves the challenge to a short origin note, and keeps self-hosting plus network boundaries concise and factual.

How to verify

• Review the rendered README and Mermaid diagram on GitHub.
• Open docs/assets/readme/webmcp-computer-demo.mp4 and confirm the brief-to-order flow.
• Run: cd web && bun test src server
• Run: cd web && bunx tsc --noEmit
• Run: cd web && bun run build
• Optional: rerun cd web && bun e2e/record-readme-demo.ts with Chrome 151+ and ffmpeg installed.

Checklist

• Tests pass locally for the touched web package: 397 passed
• Typecheck passes
• Production build passes
• No WebMCP tool contracts changed
• No secrets, account IDs, tokens, or user data are present
• No shared Worker contracts changed
• Documentation updated