When new files are being created, GH Copilot CLI often tries to read them first and outputs failed Read operations

### Describe the bug

For many prompts that result in new files being created, the harness first tries to read the still nonexistent files (probably as a safety measure); but instead of that being a hidden/silent operation, the terminal output shows "Read" errors, which makes it seem like the CLI has bugs even though it's expected behavior.

### Affected version

GitHub Copilot CLI 1.0.28

### Steps to reproduce the behavior

1. Run prompts/flows that create new files ('/openspec-propose' is the one I used this time)
2. See the "Read" errors in the terminal output.

### Expected behavior

No "Read" error should appear.

### Additional context

首先，使用報告中描述的 /openspec-propose 流程重現問題，並觀察 harness 何時輸出失敗的 Read 操作。追蹤建立新檔案時的 CLI 輸出，並確認對不存在檔案的預期讀取保持靜默，而真正的 Read 錯誤仍然可見。

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