Add an option to hide the interactive TUI startup banner

What feature would you like to see?

Please add a supported way to hide the interactive Codex CLI startup banner/header/top box that appears on a fresh codex TUI start and says OpenAI Codex.

Possible surfaces:

• config: hide_startup_banner = true
• CLI flag: --no-startup-banner

The default behavior can stay unchanged; this would be an opt-in for users who prefer a quieter TUI start or already have their terminal/session context visible elsewhere.

Current behavior

On codex-cli 0.142.5, starting the interactive CLI shows the startup chrome. codex --no-alt-screen changes terminal alternate-screen behavior and preserves scrollback, but it does not remove this banner/header.

Why this is useful

This is mostly a small UX preference, but it matters for users who launch Codex frequently, use terminal multiplexers, or keep their own session/project context in the terminal. A supported option would avoid brittle local wrapper or binary patch workarounds.

Related but not the same

#22047 requested codex exec --quiet for the non-interactive config summary banner and was closed as not planned. This request is narrower: the interactive TUI startup banner/header on fresh codex start.