feat: replace static simulations with live working demos

All playground cells now produce real output instead of just describing
what would happen:

- Widget Renderer: real streaming HTML into an iframe with progress bar
  + live bridge demo (sendPrompt/auto-resize via postMessage)
- CopilotKit: working chat stream that renders actual charts (Recharts)
  + bidirectional state sync with a live todo app
- Deep Agent: full pipeline that renders a real animated widget in an
  iframe at the end (not just JSON labels)
- MCP Skills: live design system token explorer rendering real HTML in
  iframes + interactive SVG diagram builder following skill rules
- Full Flow: chat-like demo where you pick a prompt and get a fully
  rendered visualization (dashboard/chart/architecture diagram)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jerelvelarde