Next.js App Router example (SSR-safe registration)

We have a Vite example (`examples/todo`). Next.js is where most React users will hit the SSR question: `tool()` must only run client-side.

## Task
- `examples/nextjs` with App Router: a 'use client' component using `useWebMCPTool` from `@josharsh/webmcp-react`
- Show the SSR-safe pattern + a short README section on what happens server-side (core already no-ops without `document`; document it)
- Must build in CI (add a step mirroring the todo example)

Start by comparing the existing examples/todo Vite example with the requested examples/nextjs App Router example and its 'use client' component. Document the server-side no-op behavior in the README, then mirror the todo example's CI build step. Done means the example builds in CI and demonstrates useWebMCPTool safely with SSR.

Written by the indexing model from the issue text.