Add kitchen-sink-lite app with UI kit styling and MCP server examples

[cching-openai]

Summary

• Add a new “kitchen-sink-lite” demo app that showcases the full window.openai surface using the Apps SDK UI components.
• Include Node and Python MCP server examples to serve the widget and expose simple tools (kitchen-sink-show and kitchen-sink-refresh).
• Refine the UI to align with the Apps SDK design tokens, add live widget state display, tool-call demos, host helpers, modal/close examples, embed/fetch/error demos, and an event log.
• Update README with a brief overview of the kitchen-sink-lite sample.

Why

Give external developers a concise, self-contained reference for wiring MCP tool responses to a widget, using host APIs, and seeing state/metadata flows in one place.

Notes

• Requires pnpm run build + pnpm run serve for assets, and pnpm --filter kitchen-sink-mcp-node start (or the Python server) to run locally.