Function feed 

pub fn feed(
    key: KeyEvent,
    buffer: &mut String,
    cursor: &mut usize,
    history_index: &mut Option<usize>,
    tab_state: &mut TabState,
    history: &CommandHistory,
    cwd: &Path,
    is_shell: bool,
) -> FeedResult

Feeds a single keystroke into Command Mode state.

buffer: the current typed text (after : or !). cursor: byte offset of the insertion point. history_index: current history scroll position (None = live buffer). tab_state: tracks the current completion cycle. history: the command history. cwd: current directory, used for path completions. is_shell: whether the leading sentinel was ! (not :).

Mutates buffer, cursor, and history_index in-place. Returns a FeedResult describing what the caller should do next.