Function run_builtin 

pub fn run_builtin(
    ctx: &mut ExecContext<'_>,
    cmd: &SimpleCommand,
) -> Result<(), BuiltinError>

Apply redirects: build optional stdin (Cursor over file content), stdout/stderr buffers. Then run the builtin with the effective streams; after that write stdout/stderr buffers to vfs if redirected.

Errors

Returns BuiltinError on redirect or builtin execution failure.