Open ended Generative UI example

[rabisg]

What

Adds a new example, examples/html-artifact, demonstrating open-ended generative UI with OpenUI: the model can generate a complete, self-contained HTML/CSS/JavaScript experience as the props of a single OpenUI component, rendered in a sandboxed iframe.

This is the "Open-Ended component inside a Declarative catalog" pattern from the State of Generative UI report: the model chats normally with markdown and emits an HtmlArtifact only when the user asks it to build something interactive (an app, game, simulation, or visualization). The artifact streams its raw source into a detailed-view panel on the right and switches to a rendered sandboxed iframe when the stream completes — using the existing assistant response stream, with no extra endpoint or tool call.

Changes

• examples/html-artifact — new Next.js example (derived from openui-chat) with:
  • HtmlArtifact component: compact inline preview, plus a DetailedViewPanel with Raw/Rendered tabs that auto-switches to Rendered (sandboxed iframe srcDoc) when streaming completes.
  • A minimal library (Response root, Markdown, HtmlArtifact) so emitting an artifact is a per-reply choice the model makes, not the only output shape.
  • Prompt rules and examples teaching the model when to use the artifact and how to emit self-contained, correctly escaped HTML.
  • A minimal /api/chat route (no tools) and a checked-in generated system prompt.
• packages/react-ui — export DetailedViewPanel / DetailedViewPortalTarget (and their prop types) from the package index so custom components can target the AgentInterface detailed view. No other library changes.
• Docs — new guide docs/agent/guides/open-ended-html, cross-linked from the Generative UI core-concepts page, the generative UI report blog post, and docs/public/AGENTS.md.
• Skill — new skills/openui/references/open-ended-html.md reference with a routing rule in SKILL.md, including an explicit warning not to make the artifact the library root.

Test Plan

• Not applicable (explain why)
• Verified locally

Ran the example end-to-end: plain messages (e.g. "hi") get a normal markdown reply; "build a sorting visualizer"-style prompts stream an HtmlArtifact with raw source in the panel, then render the completed document in the sandboxed iframe. Regenerated the checked-in system prompt via pnpm generate:prompt (which also validates the library through the CLI bundler). Lint and build pass for the example and