feat(commands): command-line completion for Goto/Colorize/tag commands (P3)

Upstream attaches -complete= to several commands; nuwiki had none, so <Tab>
at the : line never completed page/tag/colour names. New
autoload/nuwiki/complete.vim adds pure client-side, synchronous completers
(config + filesystem, never the LSP — so they work under vim-lsp, coc and
Neovim), wired via -complete=customlist into all command contexts:

- nuwiki#complete#pages → VimwikiGoto/NuwikiGoto (glob wiki roots for page names)
- nuwiki#complete#colors → VimwikiColorize/NuwikiColorize (color_dic keys +
  colours already used in buffer color:… spans)
- nuwiki#complete#tags → VimwikiSearchTags/GenerateTagLinks/GenerateTags (+Nuwiki)
  (scan wiki files for :tag: runs — the tag index is server-side and a
  synchronous completer can't query it, so we read the same source it indexes)

Documented divergence: no file completer for VimwikiRenameFile — nuwiki
delegates renaming to the LSP rename request (no filename arg to complete),
unlike upstream which renames itself.

Tests: complete.{pages_globs_and_filters,tags_scans_wiki_files,
colors_from_buffer_spans} (test-keymaps-vim.vim). Both harnesses green.

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

development/tests/test-keymaps-vim.vim

@@ -689,6 +689,44 @@ call s:record(
       \ 'cmd.VimwikiSplitLink_accepts_args',
       \ 'err=' . s:sl_err)
 
+" ===== Command-line completion (nuwiki#complete#*) =====
+" Pure client-side completers backing the -complete= specs on VimwikiGoto
+" (pages), VimwikiColorize (colours), and the tag commands. Driven directly
+" (no LSP) against a throwaway wiki dir.
+let s:cw = tempname()
+call mkdir(s:cw, 'p')
+call writefile(['= alpha =', 'tagged :foo:bar:'], s:cw . '/alpha.wiki')
+call writefile(['= beta ='], s:cw . '/beta.wiki')
+let s:save_root = get(g:, 'nuwiki_wiki_root', v:null)
+let g:nuwiki_wiki_root = s:cw
+
+let s:pg = nuwiki#complete#pages('a', '', 0)
+call s:record(
+      \ (index(s:pg, 'alpha') >= 0 && index(s:pg, 'beta') < 0) ? 1 : 0,
+      \ 'complete.pages_globs_and_filters',
+      \ 'got ' . string(s:pg))
+
+let s:tg = nuwiki#complete#tags('', '', 0)
+call s:record(
+      \ (index(s:tg, 'foo') >= 0 && index(s:tg, 'bar') >= 0) ? 1 : 0,
+      \ 'complete.tags_scans_wiki_files',
+      \ 'got ' . string(s:tg))
+
+if s:save_root is v:null
+  unlet g:nuwiki_wiki_root
+else
+  let g:nuwiki_wiki_root = s:save_root
+endif
+call delete(s:cw, 'rf')
+
+" Colours come from `color:NAME` spans already in the buffer.
+call s:set_buf(['<span style="color:tomato">x</span> plain'])
+let s:cl = nuwiki#complete#colors('t', '', 0)
+call s:record(
+      \ index(s:cl, 'tomato') >= 0 ? 1 : 0,
+      \ 'complete.colors_from_buffer_spans',
+      \ 'got ' . string(s:cl))
+
 " ===== Wrap up =====
 
 call add(s:results, '')