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_

一覧にある手順を使って大規模なコードベースで遅延を再現し、background-agent の完了と、grep/view を使った直接調査を比較します。次に、background-agent のスケジューリングとキャンセルのエントリポイントを追跡し、直接調査ですでに同じ範囲をカバーしている場合に冗長な処理を防ぐ、一貫したポリシーを定義して検証します。

索引モデルが issue の本文から書いたものです。