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/config-expected.txt

@@ -3,11 +3,13 @@ wiki_root=~/vimwiki
 file_extension=.wiki
 syntax=vimwiki
 log_level=warn
+diagnostic.link_severity=warn
 [sample]
 wiki_root=/tmp/base
 file_extension=.md
 syntax=vimwiki
 log_level=debug
+diagnostic.link_severity=error
 wikis[0].diary_rel_path=journal
 wikis[0].file_extension=.wiki
 wikis[0].index=home