fix(vars): E121 in get_wikilocal no-index path; add regression test

s:resolve_index referenced an undefined `a:file` (a copy-paste leftover
from wiki_root_for, whose parameter is named a:file) inside the
buffer-owning-wiki loop. Any get_wikilocal() call with no explicit index
while a wiki buffer was current — i.e. the ftplugin BufRead path — hit
`E121: undefined variable a:file`. The local is `l:file`, and the
surrounding `!empty(l:file)` guard already covers emptiness, so the
clause was both wrong and redundant; removed it.

Add development/tests/test-vars-vim.{sh,vim} covering the shim:
per-wiki-index resolution, global-default fallback, out-of-range clamp,
the no-index owning-wiki path (direct guard for this E121), and the
single-wiki shorthand. Wired into CI. 14/14 pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

autoload/vimwiki/vars.vim

@@ -21,7 +21,7 @@ function! s:resolve_index(...) abort
     for l:w in nuwiki#commands#wiki_list()
       let l:wroot = fnamemodify(expand(l:w.root), ':p')
       if l:wroot[len(l:wroot) - 1:] !=# '/' | let l:wroot .= '/' | endif
-      if a:file !=# '' && l:file[: len(l:wroot) - 1] ==# l:wroot
+      if l:file[: len(l:wroot) - 1] ==# l:wroot
         return l:n
       endif
       let l:n += 1