feat(commands): VimwikiSplitLink/VSplitLink -nargs=* signature parity (P3)

Upstream's :Vimwiki[V]SplitLink take optional reuse_other_split_window +
move_cursor flags (the link is always the one under the cursor); nuwiki's defs
took no args, so `:VimwikiSplitLink 0 1` raised E488. All eight defs (both
clients × Split/VSplit × Vimwiki/Nuwiki) are now -nargs=*, restoring signature
parity for migrated configs/scripts.

Documented divergence: the two flags are accepted but not acted on — nuwiki
always opens a fresh split and follows in it. Honoring move_cursor/reuse would
require threading a completion callback through the async LSP follow path,
disproportionate for this rarely-used arg form; the no-arg path (99% case) is
fully equivalent. The gap-doc note (which wrongly described the args as a
"target link") is corrected.

Test: cmd.VimwikiSplitLink_accepts_args (test-keymaps-vim.vim, no E488).

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

development/tests/test-keymaps-vim.vim

@@ -669,6 +669,26 @@ call s:record(
       \ 'cmd.VimwikiListChangeLvl_accepts_range_and_args',
       \ 'err=' . s:lcl_err)
 
+" ===== :VimwikiSplitLink -nargs=* =====
+" Upstream takes optional reuse/move-cursor flags; nuwiki's defs took none, so
+" `:VimwikiSplitLink 0 1` raised E488. Now -nargs=*: assert the arg parse
+" succeeds. The split opens a window (the follow itself is an LSP no-op here);
+" collapse back to one window afterward.
+call s:set_buf(['[[Target]]'])
+let s:sl_err = ''
+try
+  silent VimwikiSplitLink 0 1
+catch /E488/
+  let s:sl_err = v:exception
+catch
+  " non-E488 (e.g. no LSP attached) is expected and fine here.
+endtry
+silent! only
+call s:record(
+      \ s:sl_err ==# '' ? 1 : 0,
+      \ 'cmd.VimwikiSplitLink_accepts_args',
+      \ 'err=' . s:sl_err)
+
 " ===== Wrap up =====
 
 call add(s:results, '')