Add a runnable Vercel AI SDK example to examples/

[JoniMartin27]

examples/ has 01-http-curl.sh, 02-otel.mjs and 03-mcp.mjs, but nothing for the Vercel AI SDK (ai package), which is one of the most common ways people build TS/JS agents.

Task

Add examples/04-vercel-ai-sdk.mjs that:

1. Makes a generateText / streamText call with the Vercel AI SDK.
2. Emits a Lookspan span for it — either via the OpenAI drop-in (@lookspan/openai) under the hood, or by POSTing a span to http://127.0.0.1:3100/api/ingest (see the curl example for the payload shape).
3. Prints the trace URL so the user can open it.

Where

• New file under examples/, mirroring the style of examples/02-otel.mjs.
• Add a line to examples/README.md.

Acceptance

• node examples/04-vercel-ai-sdk.mjs produces a visible trace in a running npx lookspan dashboard.
• No secrets committed; read the key from process.env.