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

docs/get-started/configuration.md

@@ -65,11 +65,12 @@ locations for these files:
     controls over users' Gemini CLI setups.
 
 **Note on environment variables in settings:** String values within your
-`settings.json` files can reference environment variables using either
-`$VAR_NAME` or `${VAR_NAME}` syntax. These variables will be automatically
-resolved when the settings are loaded. For example, if you have an environment
-variable `MY_API_TOKEN`, you could use it in `settings.json` like this:
-`"apiKey": "$MY_API_TOKEN"`.
+`settings.json` and `gemini-extension.json` files can reference environment
+variables using either `$VAR_NAME` or `${VAR_NAME}` syntax. These variables will
+be automatically resolved when the settings are loaded. For example, if you have
+an environment variable `MY_API_TOKEN`, you could use it in `settings.json` like
+this: `"apiKey": "$MY_API_TOKEN"`. Additionally, each extension can have its own
+`.env` file in its directory, which will be loaded automatically.
 
 > **Note for Enterprise Users:** For guidance on deploying and managing Gemini
 > CLI in a corporate environment, please see the