Update extension examples (#16274)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

packages/cli/src/commands/extensions/examples/mcp-server/README.md

@@ -0,0 +1,35 @@
+# MCP Server Example
+
+This is a basic example of an MCP (Model Context Protocol) server used as a
+Gemini CLI extension. It demonstrates how to expose tools and prompts to the
+Gemini CLI.
+
+## Description
+
+The contents of this directory are a valid MCP server implementation using the
+`@modelcontextprotocol/sdk`. It exposes:
+
+- A tool `fetch_posts` that mock-fetches posts.
+- A prompt `poem-writer`.
+
+## Structure
+
+- `example.js`: The main server entry point.
+- `gemini-extension.json`: The configuration file that tells Gemini CLI how to
+  use this extension.
+- `package.json`: Helper for dependencies.
+
+## How to Use
+
+1.  Navigate to this directory:
+
+    ```bash
+    cd packages/cli/src/commands/extensions/examples/mcp-server
+    ```
+
+2.  Install dependencies:
+    ```bash
+    npm install
+    ```
+
+This example is typically used by `gemini extensions new`.