Make the source pass the CI lint

The central pipeline lints where the old inline one did not, and it
aborts on any warning. Nothing here was actually wrong: 166 of the 243
warnings were the TIC-80 API and the game's own globals being undeclared,
which is what .luacheckrc is for — the same shape impostor already uses.

The rest was trailing whitespace on 14 lines, now stripped. INPUT_KEY_X
is unused but kept: it belongs to the LEFT/RIGHT/A/B/X/Y set and removing
it would leave a gap in the mapping, so luacheck is told to allow the
family.

Verified locally by merging the sources the way the pipeline does and
running luacheck over the result: 0 warnings, 0 errors.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

inc/window/window.menu.lua

@@ -53,7 +53,7 @@ function MenuWindow.refresh_menu_items()
   table.insert(Context.menu_items, {label = "Load Game", action = MenuWindow.load_game})
   table.insert(Context.menu_items, {label = "Configuration", action = MenuWindow.configuration})
   table.insert(Context.menu_items, {label = "Exit", action = MenuWindow.exit})
-  
+
   Context.selected_menu_item = 1 -- Reset selection after refreshing
 end