feat(examples): add pi-agent-harness

[vishxrad]

Summary

Adds a new example: a web frontend that chats with the pi coding agent and renders its answers as generative UI with OpenUI. It also introduces a harnesses/ parent folder under examples/ for coding-agent harness integrations, with a matching "Harnesses" group in the docs.

What is included

• examples/harnesses/pi-agent-harness/: a single Next.js app. The /api/chat route bridges the pi SDK (@earendil-works/pi-coding-agent) to the OpenUI frontend over an OpenAI-compatible NDJSON stream. Each chat thread maps to a persistent pi AgentSession, and the agent's reasoning and tool executions are surfaced as cards in the "behind the scenes" panel.
• Docs: a new example page under "OpenUI Lang > Harnesses", plus a demo video.

Notes

• The pi SDK is ESM-only and Node-only, so it is loaded as a webpack external and the example runs with next dev --webpack.
• The agent runs with the full read/bash/edit/write toolset and the route is unauthenticated, which is fine for local use. The README and docs explain how to lock it down for networked use (PI_WEB_TOOLS=read-only, auth, loopback binding, sandboxing).
• The agent operates on a workspace directory passed at launch (pnpm dev -- /path).

Testing

Developed and verified as a standalone app against the npm-published pi SDK: token streaming, multi-turn context, and the tool and thinking cards all work, and the docs page renders locally with the new Harnesses nav. It has not been run through a full pnpm install and build inside this monorepo yet.