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)

src/index.ts

@@ -262,6 +262,7 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
     ? createAutoUpdateCheckerHook(ctx, {
         showStartupToast: isHookEnabled("startup-toast"),
         isSisyphusEnabled: pluginConfig.sisyphus_agent?.disabled !== true,
+        autoUpdate: pluginConfig.auto_update ?? true,
       })
     : null;
   const keywordDetector = isHookEnabled("keyword-detector")