Add gemini extensions link command (#7241)

Co-authored-by: Bryan Morgan <bryanmorgan@google.com>

packages/cli/src/commands/extensions/examples/context/GEMINI.md

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+# Ink Library Screen Reader Guidance
+
+When building custom components, it's important to keep accessibility in mind. While Ink provides the building blocks, ensuring your components are accessible will make your CLIs usable by a wider audience.
+
+## General Principles
+
+Provide screen reader-friendly output: Use the useIsScreenReaderEnabled hook to detect if a screen reader is active. You can then render a more descriptive output for screen reader users.
+Leverage ARIA props: For components that have a specific role (e.g., a checkbox or a button), use the aria-role, aria-state, and aria-label props on <Box> and <Text> to provide semantic meaning to screen readers.

packages/cli/src/commands/extensions/examples/context/gemini-extension.json

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+{
+  "name": "context-example",
+  "version": "1.0.0",
+  "contextFileName": "GEMINI.md"
+}

packages/cli/src/commands/extensions/examples/custom-commands/commands/fs/grep-code.toml

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+prompt = """
+Please summarize the findings for the pattern `{{args}}`.
+
+Search Results:
+!{grep -r {{args}} .}
+"""

packages/cli/src/commands/extensions/examples/custom-commands/gemini-extension.json

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+{
+  "name": "custom-commands",
+  "version": "1.0.0"
+}

packages/cli/src/commands/extensions/examples/exclude-tools/gemini-extension.json

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+{
+  "name": "excludeTools",
+  "version": "1.0.0",
+  "excludeTools": ["run_shell_command(rm -rf)"]
+}

packages/cli/src/commands/extensions/examples/mcp-server/example.ts

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+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';
+import { StdioServerTransport } from '@modelcontextprotocol/sdk/server/stdio.js';
+import { z } from 'zod';
+
+const server = new McpServer({
+  name: 'prompt-server',
+  version: '1.0.0',
+});
+
+server.registerTool(
+  'fetch_posts',
+  {
+    description: 'Fetches a list of posts from a public API.',
+    inputSchema: z.object({}).shape,
+  },
+  async () => {
+    const apiResponse = await fetch(
+      'https://jsonplaceholder.typicode.com/posts',
+    );
+    const posts = await apiResponse.json();
+    const response = { posts: posts.slice(0, 5) };
+    return {
+      content: [
+        {
+          type: 'text',
+          text: JSON.stringify(response),
+        },
+      ],
+    };
+  },
+);
+
+server.registerPrompt(
+  'poem-writer',
+  {
+    title: 'Poem Writer',
+    description: 'Write a nice haiku',
+    argsSchema: { title: z.string(), mood: z.string().optional() },
+  },
+  ({ title, mood }) => ({
+    messages: [
+      {
+        role: 'user',
+        content: {
+          type: 'text',
+          text: `Write a haiku${mood ? ` with the mood ${mood}` : ''} called ${title}. Note that a haiku is 5 syllables followed by 7 syllables followed by 5 syllables `,
+        },
+      },
+    ],
+  }),
+);
+
+const transport = new StdioServerTransport();
+await server.connect(transport);

packages/cli/src/commands/extensions/examples/mcp-server/gemini-extension.json

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+{
+  "name": "mcp-server",
+  "version": "1.0.0",
+  "mcpServers": {
+    "nodeServer": {
+      "command": "node",
+      "args": ["${extensionPath}${/}example.ts"]
+    }
+  }
+}