Background explore agents take 140+ seconds and return after agent has already found answers

### Describe the bug

The agent launched two background agents early in the investigation. Both took ~140 seconds to complete. By the time they returned results, the agent had already found most of the same information through direct grep/view calls. This consumed resources without meaningfully accelerating the investigation in this case

### Affected version

GitHub Copilot CLI 1.0.36

### Steps to reproduce the behavior

1. Ask the agent to investigate a large codebase
2. Agent launches multiple background agents
3. Agent also starts direct investigation with grep/view
4. Observe explore agents complete after the direct investigation has progressed past the agents' scope

### Expected behavior

The agent should either wait for agents before duplicating work, or cancel agents once the main thread has found the same info, or better calibrate when to use agents vs direct investigation based on complexity

### Additional context

_No response_

按照列出的步骤在大型代码库中复现延迟，并比较 background-agent 的完成情况与直接使用 grep/view 进行调查的结果。追踪 background-agent 的调度和取消入口，然后定义并验证一项一致的策略：当直接调查已经覆盖相同范围时，避免重复工作。

由索引模型根据 Issue 内容生成。