Custom agent with tools specification (run as subagent) confuses them with lack of skills

### Describe the bug

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### Affected version

_No response_

### Steps to reproduce the behavior

- Have a custom agent X with tools specification and in instructions point to using some skills
- Run the agent as the subagent of e.g. orchestrator agent
- Observe the the custom agent X confuses tools with skills and reports lack of skills available

### Expected behavior

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### Additional context

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先重現 issue 中描述的自訂代理程式 X 情境：為它提供 tools 規格，在其 instructions 中參照 skills，並將它作為 orchestrator agent 的 subagent 執行。比較 subagent 如何解讀 tools 與 skills；當指定了 tools 時，它不再回報參照的 skills 無法使用，即表示完成。

由索引模型根據 Issue 內容生成。