feat(client): send diagnostic.link_severity from both clients

Neovim adds a `diagnostic = { link_severity = 'warn' }` default so the value
is actually included in the payload. Vim reads the flat g:nuwiki_link_severity
global and nests it into the same `diagnostic` dict, keeping the two clients'
server-bound payloads identical.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

autoload/nuwiki/lsp.vim

@@ -81,6 +81,9 @@ function! s:settings() abort
         \ 'file_extension': get(g:, 'nuwiki_file_extension', '.wiki'),
         \ 'syntax':         get(g:, 'nuwiki_syntax',         'vimwiki'),
         \ 'log_level':      get(g:, 'nuwiki_log_level',      'warn'),
+        \ 'diagnostic': {
+        \   'link_severity': get(g:, 'nuwiki_link_severity', 'warn'),
+        \ },
         \ }
   " Include the per-wiki list when configured via g:nuwiki_wikis so the
   " server knows each wiki's root, diary path, etc. Without this it only