Refactor usage statistics to be a top-level setting (#1363)

This commit refactors the `usageStatisticsEnabled` setting from a sub-property of the `telemetry` configuration to a top-level setting. This change simplifies the configuration by decoupling usage statistics from the telemetry settings.

The documentation has also been updated to reflect this change.

docs/cli/configuration.md

@@ -169,7 +169,6 @@ In addition to a project settings file, a project's `.gemini` directory can cont
     - **`target`** (string): The destination for collected telemetry. Supported values are `local` and `gcp`.
     - **`otlpEndpoint`** (string): The endpoint for the OTLP Exporter.
     - **`logPrompts`** (boolean): Whether or not to include the content of user prompts in the logs.
-    - **`usageStatisticsEnabled`** (boolean): Enables or disables the collection of usage statistics. See [Usage Statistics](#usage-statistics) for more information.
   - **Example:**
     ```json
     "telemetry": {
@@ -179,6 +178,13 @@ In addition to a project settings file, a project's `.gemini` directory can cont
       "logPrompts": false
     }
     ```
+- **`usageStatisticsEnabled`** (boolean):
+  - **Description:** Enables or disables the collection of usage statistics. See [Usage Statistics](#usage-statistics) for more information.
+  - **Default:** `true`
+  - **Example:**
+    ```json
+    "usageStatisticsEnabled": false
+    ```
 
 ### Example `settings.json`:
 
@@ -201,9 +207,9 @@ In addition to a project settings file, a project's `.gemini` directory can cont
     "enabled": true,
     "target": "local",
     "otlpEndpoint": "http://localhost:4317",
-    "logPrompts": true,
-    "usageStatisticsEnabled": false
-  }
+    "logPrompts": true
+  },
+  "usageStatisticsEnabled": true
 }
 ```
 
@@ -423,18 +429,6 @@ You can opt out of usage statistics collection at any time by setting the `usage
 
 ```json
 {
-  "telemetry": {
-    "usageStatisticsEnabled": false
-  }
-}
-```
-
-You can also disable all telemetry data collection by setting the `enabled` property to `false`:
-
-```json
-{
-  "telemetry": {
-    "enabled": false
-  }
+  "usageStatisticsEnabled": false
 }
 ```