fix: warn when --add-dir would be ignored

[tibo-openai]

Add shared helper to format warnings when add-dir is incompatible with the sandbox. Surface the warning in the TUI entrypoint and document the limitation for add-dir.

[pakrym-oai]

Can we use standard mechanism we use for other startup errors (like MCP client failed to start)?

[tibo-openai]

The way the error is reported is standard across the file, for example see below (and above)

if let Err(err) = enforce_login_restrictions(&config).await {
  eprintln!("{err}");
  std::process::exit(1);
}

[pakrym-oai]

Now that you added exit() it is!