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.

packages/cli/src/ui/components/ExitPlanModeDialog.tsx

@@ -65,7 +65,7 @@ function usePlanContent(planPath: string, config: Config): PlanContentState {
       try {
         const pathError = await validatePlanPath(
           planPath,
-          config.storage.getProjectTempPlansDir(),
+          config.getPlanDirectory(),
           config.getTargetDir(),
         );
         if (ignore) return;
@@ -83,7 +83,7 @@ function usePlanContent(planPath: string, config: Config): PlanContentState {
 
         const result = await processSingleFileContent(
           planPath,
-          config.storage.getProjectTempPlansDir(),
+          config.getPlanDirectory(),
           config.getFileSystemService(),
         );