fix: run stty size via 'sh -c' with /dev/tty redirection

uiop:run-program may redirect the child's stdin, preventing stty
from querying the terminal. 'stty size < /dev/tty' explicitly
reads from the controlling terminal regardless of stdin setup.

src/backend/simple.lisp

@@ -36,7 +36,7 @@
              (sb-alien:free-alien winsize))))
       ;; stty size via subprocess — reliable on every Unix.
       (ignore-errors
-        (let* ((out (uiop:run-program '("stty" "size")
+        (let* ((out (uiop:run-program '("sh" "-c" "stty size < /dev/tty")
                                       :output :string
                                       :ignore-error-status t))
                (parts (and out (uiop:split-string