feat: add a standard CDN bundle without demo auto-start

[voidborne-d]

Summary

• add a standard CDN IIFE bundle that exposes window.PageAgent without auto-starting a demo instance
• keep the existing demo bundle as a separate build output for quick evaluation
• document both CDN entry points in the English and Chinese READMEs and developer guide

Testing

• npm run build:demo -w page-agent

Closes #433

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[gaomeng1900]

Thanks for the contribution.
I’m closing this PR because it was flagged by our review checks as high risk for insufficient human ownership.
This project does not accept bot/AI-generated contributions without substantial human involvement and review.