Add a setting to disable the startup welcome banner

Feature request

Add a settings.json option (or env var) to fully suppress the welcome banner/header box shown at the start of every interactive session — the box with the logo, version, cwd, and model info.

Context

There's already spinnerTipsEnabled: false to hide the rotating tips, and env vars like CLAUDE_CODE_HIDE_CWD to hide just the cwd line, but there's no documented way to suppress the banner box itself. --bare mode reduces startup output too, but it also disables hooks, LSP, plugin sync, memory, and other functionality — too broad a tradeoff just to skip the banner.

Proposal

A setting such as:

{
  "startupBannerEnabled": false
}

that hides the whole welcome box on session start, for users who prefer a quieter terminal on launch.