fix: resolve GitHub Actions workflow hang after task completion

- Add process.exit(0) in runner.ts for immediate termination
- Fix Timer type to ReturnType<typeof setInterval> in manager.ts
- Add .unref() to BackgroundManager polling interval
- Add cleanup() method to BackgroundManager

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

src/features/background-agent/manager.ts

@@ -57,7 +57,7 @@ export class BackgroundManager {
   private notifications: Map<string, BackgroundTask[]>
   private client: OpencodeClient
   private directory: string
-  private pollingInterval?: Timer
+  private pollingInterval?: ReturnType<typeof setInterval>
 
   constructor(ctx: PluginInput) {
     this.tasks = new Map()
@@ -287,6 +287,7 @@ export class BackgroundManager {
     this.pollingInterval = setInterval(() => {
       this.pollRunningTasks()
     }, 2000)
+    this.pollingInterval.unref()
   }
 
   private stopPolling(): void {
@@ -296,6 +297,12 @@ export class BackgroundManager {
     }
   }
 
+  cleanup(): void {
+    this.stopPolling()
+    this.tasks.clear()
+    this.notifications.clear()
+  }
+
   private notifyParentSession(task: BackgroundTask): void {
     const duration = this.formatDuration(task.startedAt, task.completedAt)