fix(background-agent): add polling mechanism for child session tracking

- Replace unreliable event-based tracking with 2-second polling
- Use SDK session.get() to detect completion (status === idle)
- Use SDK session.messages() to count tool_use parts for progress
- Auto-start polling on launch, auto-stop when no running tasks
- Resume polling on restore if running tasks exist

Fixes: Child session events not reaching plugin event handler

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

.opencode/background-tasks.json

@@ -7,7 +7,21 @@
     "description": "Explore opencode in codebase",
     "agent": "explore",
     "status": "completed",
-    "startedAt": 1765434417395,
-    "completedAt": 1765434456778
+    "startedAt": "2025-12-11T06:26:57.395Z",
+    "completedAt": "2025-12-11T06:27:36.778Z"
+  },
+  {
+    "id": "bg_392b9c9b",
+    "sessionID": "ses_4f38ebf4fffeJZBocIn3UVv7vE",
+    "parentSessionID": "ses_4f38eefa0ffeKV0pVNnwT37P5L",
+    "parentMessageID": "msg_b0c7110d2001TMBlPeEYIrByvs",
+    "description": "Test explore agent",
+    "agent": "explore",
+    "status": "running",
+    "startedAt": "2025-12-11T08:05:07.378Z",
+    "progress": {
+      "toolCalls": 0,
+      "lastUpdate": "2025-12-11T08:05:07.378Z"
+    }
   }
 ]