fix(background-agent): set default concurrency to 5

🤖 Generated with [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/features/background-agent/concurrency.ts

@@ -17,7 +17,7 @@ export class ConcurrencyManager {
     if (this.config?.providerConcurrency?.[provider]) {
       return this.config.providerConcurrency[provider]
     }
-    return this.config?.defaultConcurrency ?? Infinity
+    return this.config?.defaultConcurrency ?? 5
   }
 
   async acquire(model: string): Promise<void> {