test(config): cover link_severity in parity harnesses and docs

Both editor harnesses now emit diagnostic.link_severity and exercise a
non-default override ('error') in the sample scenario, diffed against the
shared golden. Drop the README "not wired" caveat now that the value flows
through, and document the g:nuwiki_link_severity global.

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

development/tests/test-config.lua

@@ -27,6 +27,9 @@ local function dump(payload)
   emit('file_extension=' .. tostring(payload.file_extension))
   emit('syntax=' .. tostring(payload.syntax))
   emit('log_level=' .. tostring(payload.log_level))
+  if payload.diagnostic then
+    emit('diagnostic.link_severity=' .. tostring(payload.diagnostic.link_severity))
+  end
   if payload.wikis then
     for i, w in ipairs(payload.wikis) do
       local keys = {}
@@ -53,6 +56,7 @@ config.apply({
   file_extension = '.md',
   syntax = 'vimwiki',
   log_level = 'debug',
+  diagnostic = { link_severity = 'error' },
   wikis = {
     {
       name = 'personal',