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

src/features/background-agent/manager.ts

@@ -232,7 +232,7 @@ export class BackgroundManager {
       }).catch(() => {})
     }
 
-    const message = `[BACKGROUND TASK COMPLETED] Task "${task.description}" finished in ${duration} (${toolCalls} tool calls). Use background_result with taskId="${task.id}" to get results.`
+    const message = `[BACKGROUND TASK COMPLETED] Task "${task.description}" finished in ${duration} (${toolCalls} tool calls). Use background_output with task_id="${task.id}" to get results.`
 
     const mainSessionID = getMainSessionID()
     if (!mainSessionID) {