Rewrite README to match the actual bookmarklet architecture

The README still described the old cookie-paste flow. Updated to
reflect the current design (no login/cookie, same-origin scraping,
same-tab navigation, auto-scroll), documented what's genuinely not
available (condition, sometimes brand/size) and why, and added the
live site link.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>