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>

ftplugin/vimwiki.vim

@@ -221,7 +221,11 @@ if !has('nvim')
       setlocal foldmethod=expr
       setlocal foldexpr=nuwiki#folding#expr()
       setlocal foldtext=nuwiki#folding#foldtext()
-      let b:undo_ftplugin .= ' | setlocal foldmethod< foldexpr< foldtext<'
+      " Start with every heading-block fold open; the user can still close
+      " them with `zc` and that state survives because `foldlevel` only
+      " controls the initial state, not subsequent manual fold operations.
+      setlocal foldlevel=99
+      let b:undo_ftplugin .= ' | setlocal foldmethod< foldexpr< foldtext< foldlevel<'
     endif
 
     " Text objects — vimwiki parity (matches lua/nuwiki/textobjects.lua).