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

先使用所示的 settings.json 和 /bin/date 命令重現問題，然後檢查 /statusline 路徑以及針對 showCustom 的頁尾處理。確認設定的命令是否透過 stdin 接收工作階段狀態 JSON 並被呼叫。當 footer.showCustom=true 時，命令能夠執行且其 stdout 出現在自訂狀態列中，即表示完成。

由索引模型根據 Issue 內容生成。