fix(core): reset foldlevel on entering a diff window

lua/core/autocommands.lua

@@ -66,6 +66,18 @@ vim.api.nvim_create_autocmd({ "BufReadPost" }, {
     command = 'silent! normal! g`"zv',
 })
 
+vim.api.nvim_create_autocmd("BufWinEnter", {
+    desc = "Reset foldlevel to 0 when entering a diff window."
+        .. " Vim's partial diff-state restoration on buffer re-entry"
+        .. " (e.g. via <C-o>) doesn't re-apply foldlevel=0, so"
+        .. " foldlevelstart leaks through and folds appear open.",
+    callback = function()
+        if vim.wo.diff then
+            vim.wo.foldlevel = 0
+        end
+    end,
+})
+
 vim.api.nvim_create_autocmd("FileType", {
     pattern = { "c" },
     callback = function()