fix(commands): VimwikiTable default 5 cols (parity) + map audit findings

Verification audit of the command-attribute pass confirmed all four changes
(Table, Index/TabIndex, ListChangeLvl, SplitLink) faithful with no regressions,
and surfaced three pre-existing items:

- VimwikiTable no-arg default cols was 3 vs upstream's 5 (rows 2 already
  matched, confirmed against vimwiki#tbl#create). Aligned both backing fns
  (autoload + lua) to default 5 cols.

- Mapped (not fixed here — core <CR> behavior, out of attribute scope):
  the Neovim lua follow_link_or_create double-calls definition() with dead
  code and never wraps a bare word into [[link]] (the Vim side does); and the
  Neovim Split/VSplitLink dispatch to raw definition() instead of
  follow_link_or_create. Both logged in development/vimwiki-gap.md.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

autoload/nuwiki/commands.vim

@@ -854,10 +854,10 @@ endfunction
 " Cluster B — table rewriters.
 
 function! nuwiki#commands#table_insert(...) abort
-  let l:cols = a:0 >= 1 ? str2nr(a:1) : 3
+  let l:cols = a:0 >= 1 ? str2nr(a:1) : 5
   let l:rows = a:0 >= 2 ? str2nr(a:2) : 2
   if l:cols <= 0
-    let l:cols = 3
+    let l:cols = 5
   endif
   if l:rows <= 0
     let l:rows = 2