Adds Grok Build harness example

[vishxrad]

Summary

Adds a new Grok Build harness example that connects OpenUI’s AgentInterface to xai-org/grok-build.

Linear: TH-2347 — Grok Build harness

What changed

• Added a Next.js harness under examples/harnesses/grok-build.
• Uses OpenUI’s AgentInterface and openuiChatLibrary.
• Runs Grok Build as a long-lived ACP process.
• Maintains one Grok ACP session per OpenUI thread.
• Translates ACP reasoning, tool calls, tool results, errors, and final responses into AG-UI events.
• Persists browser threads and transcripts in local storage.
• Handles cancellation, concurrent requests, retries, and restored sessions.
• Validates final OpenUI Lang before streaming it to the renderer.
• Smoothly paces OpenUI output and tool-call transitions.
• Recovers valid OpenUI candidates from retried or noisy model output.
• Renders a valid fallback card instead of exposing malformed OpenUI Lang.
• Derives the OpenUI system prompt and validation schema directly from the component library in memory, without checked-in generated artifacts.
• Adds tests covering ACP event translation, tool lifecycle handling, retries, output recovery, validation, and error serialization.

Architecture

OpenUI AgentInterface
        │
        │ AG-UI over HTTP/SSE
        ▼
Next.js chat route
        │
        │ ACP over stdio
        ▼
Grok Build agent

Grok Build exposes Agent Client Protocol events, while AgentInterface consumes supported browser streaming formats. This example therefore includes an ACP-to-AG-UI bridge that maps Grok’s native session updates into the message and tool-call lifecycle expected by OpenUI.

Future integration

The ACP bridge is currently implemented inside the example. If more ACP-compatible agents are integrated with OpenUI, we could promote this into an official server-side ACP adapter.

That integration could provide reusable handling for:

• ACP process and session lifecycle
• prompt delivery and cancellation
• reasoning and tool-call event mapping
• permissions and interactive requests
• AG-UI streaming
• OpenUI prompt attachment and final-output validation

OpenUI Lang and the renderer should remain protocol-independent; ACP support would live in the server integration layer.

Validation

• Production build
• TypeScript check
• ESLint
• 34 harness tests
• Manual Grok Build model run
• Tool-call and reasoning streaming
• Invalid OpenUI output recovery
• No generated prompt or component-schema files committed