[Bug] Cannot copy Copilot CLI output to clipboard in VSCode Server (browser-based remote environment)

# [Bug] Cannot copy Copilot CLI output to clipboard in VSCode Server (browser-based remote environment)

## Summary

When using GitHub Copilot CLI inside a **VSCode Server** environment (web-based, accessed via browser), the "copy to clipboard" functionality silently fails. There is no fallback, no error message, and no way to retrieve the output that was supposed to be copied.

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## Environment

| Item | Value |
|---|---|
| OS | Ubuntu 22.04.4 LTS |
| VSCode Server | Web-based (`--host 0.0.0.0 --port 8080`, no GUI) |
| Terminal | tmux inside VSCode Server integrated terminal |
| `$DISPLAY` | *(empty — no X11 server)* |
| `$WAYLAND_DISPLAY` | *(empty — no Wayland compositor)* |
| `xclip` | not found |
| `xsel` | not found |
| `wl-copy` | not found |
| `pbcopy` | not found (Linux) |
| TTY | `not a tty` |
| Copilot CLI version | 1.0.67 |

Relevant environment variables confirming VSCode Server context:
```
VSCODE_IPC_HOOK_CLI=/tmp/vscode-ipc-*.sock
TERM_PROGRAM=tmux
BROWSER=/usr/lib/vscode-server/bin/helpers/browser.sh
ENTRYPOINT=vscode-server --host 0.0.0.0 --port 8080 --without-connection-token
```

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## Steps to Reproduce

1. Connect to a remote machine running **VSCode Server** via browser (e.g., `http://:8080`).
2. Open an integrated terminal (which runs inside a tmux/screen session without a display server).
3. Use GitHub Copilot CLI and trigger any action that copies output to clipboard (e.g., accepting a suggested command).
4. Attempt to paste anywhere — clipboard is empty.

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## Expected Behavior

One of the following should happen:

**make copy avaliable**

In the same env, codex does;

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## Actual Behavior

The clipboard copy action **silently fails**. No output is printed, no warning is shown. The user has no way to access the content that was supposed to be copied.

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まず、VSCode Server、integrated-terminal、tmux を使用する、文書化された環境でクリップボードの失敗を再現します。次に、CLI のコピー操作と、ディスプレイもクリップボードユーティリティも利用できない場合の処理を追跡します。出力をコピーできるか、明確なフォールバックまたはエラーによって取得可能になり、暗黙に失敗しない状態になれば完了です。

索引モデルが issue の本文から書いたものです。