Update extensions docs (#16093)

Author: chrstnb

docs/extensions/best-practices.md

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+# Extensions on Gemini CLI: Best practices
+
+This guide covers best practices for developing, securing, and maintaining
+Gemini CLI extensions.
+
+## Development
+
+Developing extensions for Gemini CLI is intended to be a lightweight, iterative
+process.
+
+### Structure your extension
+
+While simple extensions can just be a few files, we recommend a robust structure
+for complex extensions:
+
+```
+my-extension/
+├── package.json
+├── tsconfig.json
+├── gemini-extension.json
+├── src/
+│   ├── index.ts
+│   └── tools/
+└── dist/
+```
+
+- **Use TypeScript**: We strongly recommend using TypeScript for type safety and
+  better tooling.
+- **Separate source and build**: Keep your source code in `src` and build to
+  `dist`.
+- **Bundle dependencies**: If your extension has many dependencies, consider
+  bundling them (e.g., with `esbuild` or `webpack`) to reduce install time and
+  potential conflicts.
+
+### Iterate with `link`
+
+Use `gemini extensions link` to develop locally without constantly reinstalling:
+
+```bash
+cd my-extension
+gemini extensions link .
+```
+
+Changes to your code (after rebuilding) will be immediately available in the CLI
+on restart.
+
+### Use `GEMINI.md` effectively
+
+Your `GEMINI.md` file provides context to the model. Keep it focused:
+
+- **Do:** Explain high-level goals and how to use the provided tools.
+- **Don't:** Dump your entire documentation.
+- **Do:** Use clear, concise language.
+
+## Security
+
+When building a Gemini CLI extension, follow general security best practices
+(such as least privilege and input validation) to reduce risk.
+
+### Minimal permissions
+
+When defining tools in your MCP server, only request the permissions necessary.
+Avoid giving the model broad access (like full shell access) if a more
+restricted set of tools will suffice.
+
+If you must use powerful tools like `run_shell_command`, consider restricting
+them to specific commands in your `gemini-extension.json`:
+
+```json
+{
+  "name": "my-safe-extension",
+  "excludeTools": ["run_shell_command(rm -rf *)"]
+}
+```
+
+This ensures that even if the model tries to execute a dangerous command, it
+will be blocked at the CLI level.
+
+### Validate inputs
+
+Your MCP server is running on the user's machine. Always validate inputs to your
+tools to prevent arbitrary code execution or filesystem access outside the
+intended scope.
+
+```typescript
+// Good: Validating paths
+if (!path.resolve(inputPath).startsWith(path.resolve(allowedDir) + path.sep)) {
+  throw new Error('Access denied');
+}
+```
+
+### Sensitive settings
+
+If your extension requires API keys, use the `sensitive: true` option in
+`gemini-extension.json`. This ensures keys are stored securely in the system
+keychain and obfuscated in the UI.
+
+```json
+"settings": [
+  {
+    "name": "API Key",
+    "envVar": "MY_API_KEY",
+    "sensitive": true
+  }
+]
+```
+
+## Releasing
+
+You can upload your extension directly to GitHub to list it in the gallery.
+Gemini CLI extensions also offers support for more complicated
+[releases](releasing.md).
+
+### Semantic versioning
+
+Follow [Semantic Versioning](https://semver.org/).
+
+- **Major**: Breaking changes (renaming tools, changing arguments).
+- **Minor**: New features (new tools, commands).
+- **Patch**: Bug fixes.
+
+### Release Channels
+
+Use git branches to manage release channels (e.g., `main` for stable, `dev` for
+bleeding edge). This allows users to choose their stability level:
+
+```bash
+# Stable
+gemini extensions install github.com/user/repo
+
+# Dev
+gemini extensions install github.com/user/repo --ref dev
+```
+
+### Clean artifacts
+
+If you are using GitHub Releases, ensure your release artifacts only contain the
+necessary files (`dist/`, `gemini-extension.json`, `package.json`). Exclude
+`node_modules` (users will install them) and `src/` to keep downloads small.