Add setting to hide prompt input border lines

Feature Request

Add a promptBorder (or similar) toggle in settings.json to hide the border lines around the prompt input area.

The statusline is already highly customizable via hooks, and settings like spinnerVerbs and effortLevel give users control over other UI elements. However, there's currently no way to hide or customize the border lines rendered around the prompt input box.

For users who prefer minimal/clean terminal UIs — especially on smaller terminals — the ability to remove this chrome would be a welcome addition.

Proposed Setting

{
  "promptBorder": false
}

Context

Working on a custom statusline and trying to minimize visual noise. The border lines are the last piece of non-configurable UI chrome.