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

Beginne damit, das Problem mit der gezeigten settings.json und dem Befehl /bin/date zu reproduzieren. Untersuche anschließend den Pfad /statusline und die Verarbeitung des Footers für showCustom. Überprüfe, ob der konfigurierte Befehl mit dem JSON des Sitzungsstatus auf stdin aufgerufen wird. Als erledigt gilt die Aufgabe, wenn der Befehl ausgeführt wird und seine stdout in der benutzerdefinierten Statuszeile erscheint, wenn footer.showCustom=true ist.

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