feat(plan): support automatic model switching for Plan Mode (#20240)

docs/reference/configuration.md

@@ -137,6 +137,12 @@ their corresponding top-level category object in your `settings.json` file.
   - **Default:** `undefined`
   - **Requires restart:** Yes
 
+- **`general.plan.modelRouting`** (boolean):
+  - **Description:** Automatically switch between Pro and Flash models based on
+    Plan Mode status. Uses Pro for the planning phase and Flash for the
+    implementation phase.
+  - **Default:** `true`
+
 - **`general.retryFetchErrors`** (boolean):
   - **Description:** Retry on "exception TypeError: fetch failed sending
     request" errors.