feat(folding): open every heading fold by default on file open

Heading-block folds were computed but all collapsed on file open,
so users had to `zR` after every `:edit`. Set `foldlevel=99` once at
ftplugin attach so the fold structure is still there (closeable with
`zc`/`zM`) but the buffer renders fully expanded.

`foldlevel` is window-local; we set it on initial attach only — not
in the Lua BufWinEnter re-apply — so closing folds with `zc` in a
window survives navigating away and back to that buffer. Split
windows still get the same value because Vim copies window-local
options to the new window when splitting.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lua/nuwiki/ftplugin.lua

@@ -35,6 +35,10 @@ local function setup_folding(bufnr, folding_mode)
   -- ftplugin). Also re-apply on any future split that views this
   -- buffer, so `<C-w>v` / `:split` preserves the fold method.
   apply()
+  -- Start with every heading-block fold open. `foldlevel` is window-local
+  -- but only sets the initial state; once the user closes folds with `zc`
+  -- those stay closed, so we don't re-apply this on BufWinEnter.
+  vim.opt_local.foldlevel = 99
   vim.api.nvim_create_autocmd('BufWinEnter', {
     buffer = bufnr,
     group = vim.api.nvim_create_augroup('nuwiki_folding_' .. bufnr, { clear = true }),