fix(compat): add vimwiki#vars shim for vimwiki-sync and vim-zettel

Third-party plugins such as vimwiki-sync and vim-zettel call
vimwiki#vars#get_wikilocal() — a function only provided by the original
vimwiki plugin. Without it, the after/ftplugin loaded by vimwiki-sync
errors on every wiki buffer open:

  E117: Unknown function: vimwiki#vars#get_wikilocal
  E121: Undefined variable: g:zettel_dir

Add autoload/vimwiki/vars.vim with lightweight stubs for the two keys
vimwiki-sync uses ('path', 'is_temporary_wiki'), reading values from
nuwiki's own g:nuwiki_wiki_root / g:nuwiki_file_extension config so
users don't have to duplicate settings. get_global() is stubbed as a
no-op for completeness.

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

autoload/vimwiki/vars.vim

@@ -0,0 +1,39 @@
+" autoload/vimwiki/vars.vim — compatibility shim for plugins that depend on
+" the original vimwiki API (e.g. vimwiki-sync, vim-zettel).
+"
+" Only the subset of `vimwiki#vars#get_wikilocal` keys that third-party
+" plugins are known to call is implemented here; add entries as needed.
+" Values are read from nuwiki's own config variables so users don't have
+" to duplicate settings.
+
+" Per-wiki local config values.
+"
+" Supported keys:
+"   path            — wiki root directory (with trailing slash)
+"   is_temporary_wiki — always 0 for a configured wiki
+"   ext / extension — file extension (default '.wiki')
+"   syntax          — syntax name (default 'vimwiki')
+function! vimwiki#vars#get_wikilocal(key, ...) abort
+  if a:key ==# 'path'
+    let l:root = expand(get(g:, 'nuwiki_wiki_root', '~/vimwiki'))
+    " Ensure trailing slash to match what the original vimwiki returns.
+    return l:root =~# '/$' ? l:root : l:root . '/'
+
+  elseif a:key ==# 'is_temporary_wiki'
+    return 0
+
+  elseif a:key ==# 'ext' || a:key ==# 'extension'
+    let l:ext = get(g:, 'nuwiki_file_extension', '.wiki')
+    return l:ext[0] ==# '.' ? l:ext : '.' . l:ext
+
+  elseif a:key ==# 'syntax'
+    return get(g:, 'nuwiki_syntax', 'vimwiki')
+
+  endif
+  return ''
+endfunction
+
+" Global (non-wiki-specific) config values.
+function! vimwiki#vars#get_global(key) abort
+  return ''
+endfunction