feat(cron): make cron tools opt-in via experimental settings

Change cron/loop tools from opt-out to opt-in. Cron tools are now
disabled by default and can be enabled via:
- settings.json: { "experimental": { "cron": true } }
- Environment variable: QWEN_CODE_ENABLE_CRON=1

This ensures experimental features are explicitly enabled by users
who want to try them.

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

packages/cli/src/ui/hooks/useGeminiStream.test.tsx

@@ -204,7 +204,7 @@ describe('useGeminiStream', () => {
         .mockReturnValue(contentGeneratorConfig),
       getMaxSessionTurns: vi.fn(() => 50),
       getArenaAgentClient: vi.fn(() => null),
-      isCronDisabled: vi.fn(() => true),
+      isCronEnabled: vi.fn(() => false),
       getCronScheduler: vi.fn(() => null),
     } as unknown as Config;
     mockOnDebugMessage = vi.fn();

packages/cli/src/ui/hooks/useGeminiStream.ts

@@ -1643,7 +1643,7 @@ export const useGeminiStream = (
 
   // Start the scheduler on mount, stop on unmount
   useEffect(() => {
-    if (config.isCronDisabled()) return;
+    if (!config.isCronEnabled()) return;
     const scheduler = config.getCronScheduler();
     scheduler.start((job: { prompt: string }) => {
       cronQueueRef.current.push(job.prompt);