Add extension settings to be requested on install (#9802)

docs/extensions/index.md

@@ -177,6 +177,40 @@ When Gemini CLI starts, it loads all the extensions and merges their
 configurations. If there are any conflicts, the workspace configuration takes
 precedence.
 
+### Settings
+
+Extensions can define settings that the user will be prompted to provide upon
+installation. This is useful for things like API keys, URLs, or other
+configuration that the extension needs to function.
+
+To define settings, add a `settings` array to your `gemini-extension.json` file.
+Each object in the array should have the following properties:
+
+- `name`: A user-friendly name for the setting.
+- `description`: A description of the setting and what it's used for.
+- `envVar`: The name of the environment variable that the setting will be stored
+  as.
+
+**Example**
+
+```json
+{
+  "name": "my-api-extension",
+  "version": "1.0.0",
+  "settings": [
+    {
+      "name": "API Key",
+      "description": "Your API key for the service.",
+      "envVar": "MY_API_KEY"
+    }
+  ]
+}
+```
+
+When a user installs this extension, they will be prompted to enter their API
+key. The value will be saved to a `.env` file in the extension's directory
+(e.g., `<home>/.gemini/extensions/my-api-extension/.env`).
+
 ### Custom commands
 
 Extensions can provide [custom commands](../cli/custom-commands.md) by placing