feat(core): mvp port planner to subagent.

docs/reference/configuration.md

@@ -1157,6 +1157,11 @@ their corresponding top-level category object in your `settings.json` file.
   - **Default:** `true`
   - **Requires restart:** Yes
 
+- **`experimental.plannerSubagent`** (boolean):
+  - **Description:** Use the new planner subagent for plan mode.
+  - **Default:** `false`
+  - **Requires restart:** Yes
+
 - **`experimental.enableAgents`** (boolean):
   - **Description:** Enable local and remote subagents. Warning: Experimental
     feature, uses YOLO mode for subagents