cc/S in the prompt (vim mode) leaves a stray prompt icon that must be backspaced #614

Description

With prompt_vim_mode = true, clearing the query from normal mode with cc (or S) leaves a stray copy of the prompt icon (e.g. 🪿) in the input bar that has to be manually backspaced out.

Steps to reproduce

1. Set prompt_vim_mode = true.
2. Open the picker and type a query.
3. Press <Esc> to enter normal mode, then cc (or S).

Expected: the query is cleared and the prompt icon remains as the prefix.
Actual: the line is fully cleared and a literal prompt icon is left behind that must be backspaced.

Root cause

The input bar is a buftype=prompt buffer with the prompt set via prompt_setprompt (lua/fff/picker_ui/ui_creator.lua). The prompt icon is protected prompt text at the start of the line. cc/S delete the entire line including the prompt, and the prompt buffer then re-inserts the icon as plain literal text.