feat(edit): stop forcing VISUAL to the terminal editor

VISUAL was hardwired to $EDITOR (nvim), so 'edit --visual' would launch a
terminal editor detached with no tty. Comment it out so --visual falls
through to the GUI fallback chain, and let users set a real GUI editor via
local.fish. Update the editor-variable docs to match.

docs/fish-config.md

@@ -159,7 +159,8 @@ Tools that respect XDG are directed to these paths rather than polluting $HOME.
 ## Editor and Pager
 
     EDITOR      nvim (falls back to vi if nvim is absent)
-    VISUAL      same as EDITOR
+    VISUAL      unset by default; set a GUI editor via local.fish (the edit
+                function falls back to a GUI chain when VISUAL is empty)
     SUDO_EDITOR same as EDITOR
     PAGER       ov (falls back to less)