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>

lua/nuwiki/commands.lua

@@ -962,7 +962,7 @@ end
 -- Cluster B — table rewriters.
 
 function M.table_insert(cols, rows)
-  cols = tonumber(cols) or 3
+  cols = tonumber(cols) or 5
   rows = tonumber(rows) or 2
   local p = pos_args()[1]
   p.cols = cols