Nimble Web Search — Mastra tool

Copy this into a Mastra agent to give it Nimble web search.
Add Nimble web search to this Mastra project.

1. Create `src/tools/nimble-web-search.ts`: a Mastra tool (`createTool`) that calls Nimble's Web Search Agent — `POST https://sdk.nimbleway.com/v2/search` with header `Authorization: Bearer $NIMBLE_API_KEY` — with a 15s timeout and a typed error on failure. Return `{ answer, sources }`.

2. Register it on an agent:

```ts
import { Agent } from "@mastra/core/agent";
import { anthropic } from "@ai-sdk/anthropic";
import { nimbleWebSearchTool } from "./tools/nimble-web-search";

const agent = new Agent({
  name: "research-agent",
  instructions: "Use nimble-web-search for anything needing current information, and cite sources.",
  model: anthropic("claude-sonnet-5"),
  tools: { nimbleWebSearchTool },
});

const result = await agent.generate("What is PydanticAI?");
console.log(result.text);
```

3. Add `NIMBLE_API_KEY` to `.env`.

Follow this repo's CLAUDE.md if one exists, and run the tests after wiring it in.