Add external editor ($EDITOR) handoff for long text

[AlexSkrypnyk]

Summary

Let the textarea widget open the user's configured $EDITOR for long multi-line input and capture the result on return.

Details

Inline multi-line editing is fine for a few lines but painful for paragraphs or structured text.

Add an opt-in option on the textarea widget (or a companion editor widget) that suspends the TUI, opens the user's $EDITOR to compose the content, and captures the saved buffer back into the answer when the editor exits.

Acceptance criteria

• An opt-in option suspends the TUI, launches the $EDITOR, and restores the TUI on exit.
• The saved buffer becomes the field value; an empty or aborted edit is handled gracefully.
• The terminal state is restored cleanly whether the editor exits normally or is cancelled.
• When no editor is available or the run is headless, the field falls back to inline textarea behaviour.
• Unit tests cover the launch/capture path with a stubbed editor and the fallback path.
• The option is documented in the README textarea section.