refactor(background-task): unify background_result and background_status into background_output tool

- Merge background_status into background_output with block parameter
- Replace background_result references with background_output throughout codebase
- Update tool descriptions to reflect new unified API
- Remove background-tasks.json (memory-based only)
- Simplify notification messages and tool usage instructions

.opencode/background-tasks.json

@@ -1,27 +0,0 @@
-[
-  {
-    "id": "bg_wzsdt60b",
-    "sessionID": "ses_4f3e89f0dffeooeXNVx5QCifse",
-    "parentSessionID": "ses_4f3e8d141ffeyfJ1taVVOdQTzx",
-    "parentMessageID": "msg_b0c172ee1001w2B52VSZrP08PJ",
-    "description": "Explore opencode in codebase",
-    "agent": "explore",
-    "status": "completed",
-    "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"
-    }
-  }
-]