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/vimwiki-gap.md

@@ -270,11 +270,26 @@ fix site.
   half is done: all four defs now use upstream's `-count=0` instead of bare
   `-count`; `wiki_index`/`wiki_tab_index` already map the count to the wiki
   number.)_
-- [ ] **No `-complete=` specs** _(new)_ — upstream attaches command-line completion
-  to `VimwikiGoto` (links), `VimwikiRenameFile` (files), `VimwikiColorize`
-  (colours), and the tag commands. nuwiki defines none, so `<Tab>` completion of
-  page / tag / colour names at the `:` line is unavailable. _Fix:_
-  `ftplugin/vimwiki.vim` (both branches) — VimL completers or LSP-backed.
+- [x] **`-complete=` specs** _(new; done 2026-05-31 command-attribute pass)_ —
+  upstream attaches command-line completion to several commands; nuwiki had
+  none, so `<Tab>` at the `:` line never completed page / tag / colour names.
+  _Fix:_ new `autoload/nuwiki/complete.vim` with pure client-side, synchronous
+  completers (config + filesystem, never the LSP, so they work under vim-lsp,
+  coc and Neovim alike), wired via `-complete=customlist,…` into all branches:
+  - `nuwiki#complete#pages` → `VimwikiGoto`/`NuwikiGoto` (globs every wiki
+    root's `*<ext>` files → root-relative page names).
+  - `nuwiki#complete#colors` → `VimwikiColorize`/`NuwikiColorize` (keys from a
+    configured `color_dic` + colours already used in buffer `color:…` spans).
+  - `nuwiki#complete#tags` → `VimwikiSearchTags`/`GenerateTagLinks`/
+    `GenerateTags` (+ Nuwiki forms) — scans the wikis' files for `:tag:` runs
+    (the tag index lives in the server, which a synchronous completer can't
+    query; the file scan reads the same source the server indexes).
+  **Divergence (documented):** no file completer for `VimwikiRenameFile` —
+  upstream completes a new-name arg because it renames itself, but nuwiki
+  delegates renaming to the LSP rename request (`:LspRename`/coc), which drives
+  its own prompt and takes no filename argument. Covered by
+  `complete.pages_globs_and_filters`, `complete.tags_scans_wiki_files`,
+  `complete.colors_from_buffer_spans` (`test-keymaps-vim.vim`).
 - [x] **Minor command-attribute gaps** _(new)_ — all now real, not cosmetic:
   `VimwikiNormalizeLink` is `-nargs=?` and with `1` wraps the `'<`/`'>` selection
   (`wrap_visual_as_wikilink`, both clients; the `x`-mode `+` passes it too);