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-vim.vim

@@ -15,6 +15,9 @@ function! s:dump(payload) abort
   call add(s:lines, 'file_extension=' . a:payload.file_extension)
   call add(s:lines, 'syntax=' . a:payload.syntax)
   call add(s:lines, 'log_level=' . a:payload.log_level)
+  if has_key(a:payload, 'diagnostic')
+    call add(s:lines, 'diagnostic.link_severity=' . a:payload.diagnostic.link_severity)
+  endif
   if has_key(a:payload, 'wikis')
     let l:i = 0
     for l:w in a:payload.wikis
@@ -28,7 +31,7 @@ endfunction
 
 " Scenario 1: defaults (no globals set).
 unlet! g:nuwiki_wiki_root g:nuwiki_file_extension g:nuwiki_syntax
-unlet! g:nuwiki_log_level g:nuwiki_wikis
+unlet! g:nuwiki_log_level g:nuwiki_wikis g:nuwiki_link_severity
 call add(s:lines, '[default]')
 call s:dump(nuwiki#lsp#settings())
 
@@ -37,6 +40,7 @@ let g:nuwiki_wiki_root      = '/tmp/base'
 let g:nuwiki_file_extension = '.md'
 let g:nuwiki_syntax         = 'vimwiki'
 let g:nuwiki_log_level      = 'debug'
+let g:nuwiki_link_severity  = 'error'
 let g:nuwiki_wikis = [
       \ { 'name': 'personal', 'root': '/tmp/personal',
       \   'file_extension': '.wiki', 'index': 'home',