feat(config): bridge setup({wikis}) to g:nuwiki_wikis for VimL plugins

A Lua-only `setup({ wikis = … })` config is invisible to the VimL side:
the buffer helpers in autoload/nuwiki/config.vim and the vimwiki compat
shim (autoload/vimwiki/vars.vim, which third-party plugins like
vimwiki-sync call via vimwiki#vars#get_wikilocal) read g:nuwiki_wikis. So
a Neovim user who migrated from g:vimwiki_list to native setup({wikis})
would break vimwiki-sync (it'd resolve the default ~/vimwiki).

setup() now publishes the resolved wiki list to g:nuwiki_wikis when the
config came from setup() opts (the only case VimL can't already see —
g:nuwiki_wikis / g:vimwiki_list configs are global and visible). Verified
vimwiki#vars#get_wikilocal('path', n) returns the right roots from a
pure-Lua setup() config. test-global-shorthand grows two bridge checks
(17). Goldens + keymap harnesses green.

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

development/tests/test-global-shorthand.lua

@@ -71,6 +71,16 @@ vim.g.nuwiki_wikis = nil
 vim.g.vimwiki_list = nil
 vim.g.vimwiki_toc_header_level = nil
 
+-- ----- setup({ wikis }) bridges the Lua config to VimL (g:nuwiki_wikis) -----
+-- So buffer-side VimL helpers + the vimwiki-sync compat shim see the same
+-- wikis as the LSP, even though setup() config lives only in Lua.
+require('nuwiki').setup({ wikis = { { root = '/tmp/a', name = 'A' }, { root = '/tmp/b' } } })
+check('setup() bridges to g:nuwiki_wikis',
+  vim.g.nuwiki_wikis ~= nil and #vim.g.nuwiki_wikis == 2)
+check('bridged wiki root visible to VimL',
+  vim.g.nuwiki_wikis and vim.g.nuwiki_wikis[1].root == '/tmp/a')
+vim.g.nuwiki_wikis = nil
+
 local f = assert(io.open(os.getenv('NUWIKI_GS_OUT'), 'w'))
 f:write(table.concat(out, '\n') .. '\n')
 f:close()