docs(readme): product-page README with screenshots and demo links

[tnfssc]

Summary

Rewrites the README as a product page:

• the marquee image and badges for Add to Chrome, the demo and the example PR;
• a short pitch;
• before/after and screenshot sections that use store/screenshots/;
• a feature list;
• an honest "Is it safe?" section;
• a 10-second try-it path.

The detailed behavior, SSO boundaries, requirements, scroll sync and execution boundary sections move verbatim to .docs/behavior.md, linked from the README and .docs/README.md. Development instructions stay in the README.

One factual fix while moving: blob and PR previews package all repository assets, public and private, as data URLs (target: 'sandbox-private' in both entrypoints). The old README said public assets are rewritten to jsDelivr.

Docs only; no code changes.

[coderabbitai]

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