Add wiki-explorer-server example (#122)

Interactive Wikipedia link graph visualization using force-graph with:
- Force-directed graph layout for exploring page connections
- Node expansion to reveal first-degree Wikipedia links
- Visual state tracking (blue=default, green=expanded, red=error)
- Direct browser access to Wikipedia pages

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

examples/wiki-explorer-server/README.md

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+# Example: Wiki Explorer
+
+Visualizes Wikipedia link graphs using a force-directed layout. Explore how Wikipedia pages are connected by expanding nodes to reveal first-degree links.
+
+<table>
+  <tr>
+    <td><a href="https://modelcontextprotocol.github.io/ext-apps/screenshots/wiki-explorer-server/01-zoomed.png"><img src="https://modelcontextprotocol.github.io/ext-apps/screenshots/wiki-explorer-server/01-zoomed.png" alt="Zoomed" width="100%"></a></td>
+    <td><a href="https://modelcontextprotocol.github.io/ext-apps/screenshots/wiki-explorer-server/02-pop-up.png"><img src="https://modelcontextprotocol.github.io/ext-apps/screenshots/wiki-explorer-server/02-pop-up.png" alt="Pop-up" width="100%"></a></td>
+    <td><a href="https://modelcontextprotocol.github.io/ext-apps/screenshots/wiki-explorer-server/03-expanded-graph.png"><img src="https://modelcontextprotocol.github.io/ext-apps/screenshots/wiki-explorer-server/03-expanded-graph.png" alt="Expanded graph" width="100%"></a></td>
+  </tr>
+</table>
+
+## Features
+
+- **Force-directed graph visualization**: Interactive graph powered by [`force-graph`](https://github.com/vasturiano/force-graph)
+- **Node expansion**: Click any node to expand and see all pages it links to
+- **Visual state tracking**: Nodes change color based on state (blue = default, green = expanded, red = error)
+- **Direct page access**: Open any Wikipedia page in your browser
+
+## Running
+
+1. Install dependencies:
+
+   ```bash
+   npm install
+   ```
+
+2. Build and start the server:
+
+   ```bash
+   npm run start:http  # for Streamable HTTP transport
+   # OR
+   npm run start:stdio  # for stdio transport
+   ```
+
+3. View using the [`basic-host`](https://github.com/modelcontextprotocol/ext-apps/tree/main/examples/basic-host) example or another MCP Apps-compatible host.
+
+### Tool Input
+
+To test the example, call the `get-first-degree-links` tool with a Wikipedia URL:
+
+```json
+{
+  "url": "https://en.wikipedia.org/wiki/Graph_theory"
+}
+```
+
+Click nodes in the graph to **Open** (view in browser) or **Expand** (visualize linked pages).
+
+## Architecture
+
+### Server (`server.ts`)
+
+MCP server that fetches Wikipedia pages and extracts internal links.
+
+Exposes one tool:
+
+- `get-first-degree-links` - Returns links to other Wikipedia pages from a given page
+
+### App (`src/mcp-app.ts`)
+
+Vanilla TypeScript app using force-graph for visualization that:
+
+- Receives tool inputs via the MCP App SDK
+- Renders an interactive force-directed graph
+- Supports node expansion to explore link relationships