Add the visual WebMCP Workbench - #14

Summary

• add an optional, development-only Shadow DOM Workbench that agents launch without changing the target project
• compare approved manifest tools with live imperative and declarative registrations
• provide schema-driven arguments, valid and invalid example presets, structured results, bounded session history, and confirmation for every non-read-only call
• distinguish Native from Simulated evidence explicitly so portable previews never count as browser verification
• support responsive desktop, tablet, and phone layouts, keyboard use, reduced motion, dark mode, strict CSP, and Trusted Types
• provision a pinned Playwright runtime outside the target project when needed and clean up the temporary browser session on exit

Verification

• npm run check — 40/40 passing
• node tests/workbench-browser.mjs chromium firefox — desktop, tablet, 360px, dark/light, reduced-motion, simulated transport, native-shaped transport, lifecycle, contrast, focus, overflow, strict CSP, and Trusted Types passing
• headed launcher smoke under Xvfb passing
• WebKit downloaded, but no runtime result is claimed because VD-FW lacks its required GTK/GStreamer host libraries

Safety

The Workbench is never wired into a production entry or bundle. Simulated calls are visibly labeled and cannot change a manifest tool to verified.