docs: Add Gemma 4 routing instructions to README and docs

docs/cli/gemma4-routing.md

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+# Gemma 4 Routing
+
+Gemini CLI allows you to seamlessly route your requests to Gemma 4 models. When
+enabled, requests that would normally be sent to standard Gemini Pro and Flash
+models are automatically redirected to your chosen Gemma 4 variant.
+
+## Configuration
+
+You can enable Gemma 4 routing using the CLI settings or by modifying your
+`settings.json` file.
+
+### Via Settings UI
+
+1. Open the settings dialog by running `/settings`.
+2. Navigate to the **Model** section.
+3. Locate the **Gemma 4 Variant** setting.
+4. Select your preferred model:
+   - `gemma-4-26b-a4b-it` (Gemma 4 26B A4B IT)
+   - `gemma-4-31b-it` (Gemma 4 31B IT)
+5. Save the settings and restart the CLI if prompted.
+
+### Via `settings.json`
+
+You can also set this directly in your `.gemini/settings.json` file:
+
+```json
+{
+  "model": {
+    "gemma4Variant": "gemma-4-31b-it"
+  }
+}
+```
+
+## How it works
+
+When a `gemma4Variant` is selected, Gemini CLI intercepts model resolution:
+
+- Requests for `gemini-pro`, `gemini-flash`, and their associated aliases (like
+  `auto`, `pro`, `flash`) are routed to the selected Gemma 4 model.
+- The **router model** (`flash-lite`) remains unaffected and continues to use
+  `gemini-2.5-flash-lite`. This ensures fast, lightweight background routing
+  tasks continue to operate optimally.
+- If you do not have preview model access, the CLI normally falls back to stable
+  models; however, the Gemma 4 variant will still take precedence for Pro and
+  Flash targets.
+
+To disable Gemma 4 routing, simply remove the `gemma4Variant` configuration from
+your settings or set it to `undefined`/empty in the UI.