feat(vim): conceal markup delimiters when conceallevel=2

Set conceallevel=2 and concealcursor=nc on .wiki buffers and add
contained `conceal` matches for the bold/italic/code delimiters and
wikilink brackets/targets. The static-fallback rendering now mirrors
what the Neovim semantic-token path produced: *bold* shows as bold
without the asterisks, [[target|desc]] shows just "desc", etc.

The wikilink region uses an inline `contains=` (no line continuation)
because some Vim builds (notably Homebrew Vim 9.2) raise E10 on the
leading `\\ contains=...` form even though the surrounding region
parses fine.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ftplugin/vimwiki.vim

@@ -18,8 +18,10 @@ setlocal comments=:%%
 setlocal formatoptions+=ron
 setlocal suffixesadd=.wiki
 setlocal iskeyword+=-
+setlocal conceallevel=2
+setlocal concealcursor=nc
 
-let b:undo_ftplugin = 'setlocal commentstring< comments< formatoptions< suffixesadd< iskeyword<'
+let b:undo_ftplugin = 'setlocal commentstring< comments< formatoptions< suffixesadd< iskeyword< conceallevel< concealcursor<'
 
 if !has('nvim')
   " ===== Plain Vim path =====