feat(auto-update-checker): implement background auto-update with configurable pinning

- Run update check in background after startup (non-blocking)
- Auto-update pinned versions in config file when newer version available
- Add auto_update config option to disable auto-updating
- Properly invalidate package cache after config update
- Scoped regex replacement to avoid editing outside plugin array

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

assets/oh-my-opencode.schema.json

@@ -1222,6 +1222,9 @@
           "type": "boolean"
         }
       }
+    },
+    "auto_update": {
+      "type": "boolean"
     }
   }
 }