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>

lua/nuwiki/config.lua

@@ -50,6 +50,13 @@ M.defaults = {
   -- provider (Neovim 0.11+ wires it automatically). `'expr'` falls
   -- back to a regex-based `foldexpr`. `'off'` skips folding setup.
   folding = 'lsp',
+
+  -- Broken-link diagnostic severity. Sent to the server, which downgrades
+  -- or suppresses the diagnostic accordingly.
+  --   'off' | 'hint' | 'warn' | 'error'
+  diagnostic = {
+    link_severity = 'warn',
+  },
 }
 
 M.options = vim.deepcopy(M.defaults)