statusLine.command is not executed or rendered when footer.showCustom=true

### Describe the bug

With `footer.showCustom=true` and `statusLine.command` set to a valid executable, Copilot CLI 1.0.43 on macOS reads `settings.json` for built-in footer items, but never executes the custom status line command and never renders custom output.

There is no visible error in the TUI.

### Affected version

GitHub Copilot CLI 1.0.43

### Steps to reproduce

1. Put this in `~/.copilot/settings.json`:

```json
{
"statusLine": {
"type": "command",
"command": "/bin/date",
"padding": 1
},
"footer": {
"showCustom": true,
"showBranch": true,
"showAgent": true
}
}
```

2. Start a fresh `copilot` session in a new terminal.
3. Verify that built-in footer settings are applied (for example, `showBranch` affects the footer, which shows `settings.json` is being read).
4. Run `/statusline` and verify that `custom` is enabled.
5. Observe that the custom output never appears in the status line.

### Additional verification

I also replaced `/bin/date` with a trivial local shell script that:
- writes a timestamp to a file
- reads stdin
- prints a short string to stdout

That script also never ran: no file was created and no custom output was rendered.

### Expected behavior

When `footer.showCustom=true`, the configured `statusLine.command` should execute, receive the current session status JSON on stdin, and render its stdout in the custom status line area.

### Actual behavior

Built-in footer/status line settings are applied, but the custom `statusLine.command` appears not to execute at all and nothing is rendered for the custom status line item.

### Environment

- OS: macOS
- Copilot CLI: 1.0.43

まず、示されている settings.json と /bin/date コマンドで問題を再現し、次に /statusline パスと showCustom のフッター処理を調べます。設定されたコマンドがセッションステータスの JSON を stdin で受け取って呼び出されるか確認します。footer.showCustom=true のときにコマンドが実行され、その stdout がカスタムステータス行に表示されれば完了です。

索引モデルが issue の本文から書いたものです。