refactor: correctness fixes, API modernization, and cleanup

lua/core/mappings.lua

@@ -3,7 +3,7 @@ vim.keymap.set("n", "tn", vim.cmd.tabnew)
 vim.keymap.set("n", "tq", vim.cmd.tabclose)
 
 -- Clipboard
-if vim.env.TMUX and vim.fn.executable("tmux") then
+if vim.env.TMUX ~= nil and vim.fn.executable("tmux") == 1 then
     vim.keymap.set(
         { "n", "x" },
         "<leader>y",

lua/core/options.lua

@@ -26,6 +26,7 @@ vim.opt.foldlevel = 99
 vim.opt.foldlevelstart = 99
 vim.opt.foldmethod = "indent"
 vim.opt.foldignore = ""
+---@diagnostic disable-next-line: undefined-field
 vim.opt.completeopt:append({
     "menu",
     "menuone",