feat: implement extensible plan mode with custom directory configuration

- Adds 'general.plan' configuration object for plan settings (directory).
- Updates 'experimental.plan' to a boolean flag for enablement.
- Implements dynamic high-priority policy for custom plan directories in core.
- Adds migration logic for previous configuration formats.
- Updates documentation and schema.

docs/get-started/configuration.md

@@ -142,6 +142,10 @@ their corresponding top-level category object in your `settings.json` file.
   - **Default:** `false`
   - **Requires restart:** Yes
 
+- **`general.plan`** (object):
+  - **Description:** Configuration for planning features.
+  - **Default:** `{}`
+
 - **`general.enablePromptCompletion`** (boolean):
   - **Description:** Enable AI-powered prompt completion suggestions while
     typing.