feat: add empty fish_mode_prompt stub to suppress vi-mode prefix

functions/fish_mode_prompt.fish

@@ -0,0 +1,16 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+#
+# SYNOPSIS
+#   fish_mode_prompt
+#
+# DESCRIPTION
+#   Empty override. Suppresses fish's built-in vi-mode prefix ([N]/[I]/etc.)
+#   that would prepend to the prompt line and break the two-line nim layout.
+#   Vi-mode display is handled inside fish_prompt itself.
+#
+# EXAMPLE
+#   # Rendered automatically by fish; not called directly.
+
+function fish_mode_prompt
+end