Docs: Add changelog for v.0.15.0 (#13276)

docs/cli/model-routing.md

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+## Model Routing
+
+Gemini CLI includes a model routing feature that automatically switches to a
+fallback model in case of a model failure. This feature is enabled by default
+and provides resilience when the primary model is unavailable.
+
+## How it Works
+
+Model routing is not based on prompt complexity, but is a fallback mechanism.
+Here's how it works:
+
+1.  **Model Failure:** If the currently selected model fails to respond (for
+    example, due to a server error or other issue), the CLI will initiate the
+    fallback process.
+
+2.  **User Consent:** The CLI will prompt you to ask if you want to switch to
+    the fallback model. This is handled by the `fallbackModelHandler`.
+
+3.  **Fallback Activation:** If you consent, the CLI will activate the fallback
+    mode by calling `config.setFallbackMode(true)`.
+
+4.  **Model Switch:** On the next request, the CLI will use the
+    `DEFAULT_GEMINI_FLASH_MODEL` as the fallback model. This is handled by the
+    `resolveModel` function in
+    `packages/cli/src/zed-integration/zedIntegration.ts` which checks if
+    `isInFallbackMode()` is true.
+
+## Configuration
+
+Model routing is controlled by the `useModelRouter` setting in your
+`settings.json` file.
+
+- **`"experimental.useModelRouter": true` (Default):** Enables the model
+  routing/fallback feature.
+
+- **`"experimental.useModelRouter": false`:** Disables the model
+  routing/fallback feature. If a model fails, the CLI will not attempt to switch
+  to a fallback model.
+
+### Model Selection Precedence
+
+The model used by Gemini CLI is determined by the following order of precedence:
+
+1.  **`--model` command-line flag:** A model specified with the `--model` flag
+    when launching the CLI will always be used.
+2.  **`GEMINI_MODEL` environment variable:** If the `--model` flag is not used,
+    the CLI will use the model specified in the `GEMINI_MODEL` environment
+    variable.
+3.  **`model.name` in `settings.json`:** If neither of the above are set, the
+    model specified in the `model.name` property of your `settings.json` file
+    will be used.
+4.  **Default Model:** If none of the above are set, the default model will be
+    used. The default model is determined by the `useModelRouter` setting:
+    - If `useModelRouter` is `true`, the default model is `"auto"`.
+    - If `useModelRouter` is `false`, the default model is the standard Gemini
+      model.