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/vscode-ide-companion/schemas/settings.schema.json

@@ -1445,9 +1445,15 @@
       }
     },
     "experimental": {
-      "description": "Setting to enable experimental features",
+      "description": "Settings to enable experimental features.",
       "type": "object",
-      "properties": {}
+      "properties": {
+        "cron": {
+          "description": "Enable in-session cron/loop tools (experimental). When enabled, the model can create recurring prompts using cron_create, cron_list, and cron_delete tools. Can also be enabled via QWEN_CODE_ENABLE_CRON=1 environment variable.",
+          "type": "boolean",
+          "default": false
+        }
+      }
     },
     "$version": {
       "type": "number",