fix(background-task): pass config to BackgroundManager for concurrency limits

The background_task config (providerConcurrency, modelConcurrency, etc.)
was not being passed to BackgroundManager, causing all models to use
the hardcoded default limit of 5 instead of user-configured values.

src/index.ts

@@ -205,7 +205,7 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
 
   const taskResumeInfo = createTaskResumeInfoHook();
 
-  const backgroundManager = new BackgroundManager(ctx);
+  const backgroundManager = new BackgroundManager(ctx, pluginConfig.background_task);
 
   const atlasHook = isHookEnabled("atlas")
     ? createAtlasHook(ctx, { directory: ctx.directory, backgroundManager })