docs(client): document link-follow commands on both editors + tab-drop

The split/tab link-follow commands are no longer Neovim-only; document
:Nuwiki/Vimwiki{Split,VSplit,Tabnew,TabDrop,GoBack}Link and note that
<C-S-CR> follows a link in a tab reusing an existing one. Tick parity
gap P1 #1 in the tracking doc.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

development/vimwiki-gap.md

@@ -15,11 +15,14 @@ fix site.
 
 ## P1 — Core workflow gaps (fix first)
 
-- [ ] **Vim client split/tab link-follow commands** — `VimwikiSplitLink`,
+- [x] **Vim client split/tab link-follow commands** — `VimwikiSplitLink`,
   `VimwikiVSplitLink`, `VimwikiGoBackLink`, `VimwikiTabnewLink`,
   `VimwikiTabDropLink` exist only in the Neovim branch. Vim users lose
   split-window link following and back-navigation.
-  _Fix:_ `ftplugin/vimwiki.vim` (Vim branch, ~lines 30–175).
+  _Fix:_ `ftplugin/vimwiki.vim` (Vim branch) — commands added + `:Nuwiki*`
+  aliases; `follow_link_drop()` helper in `autoload/nuwiki/commands.vim`;
+  true `:tab drop` tab-reuse wired in both clients (`lua/nuwiki/commands.lua`
+  `open_uri` `'tabdrop'` case); `<C-S-CR>` repointed to tab-drop.
 - [ ] **`gl<symbol>` change-symbol mappings** — upstream's one-key bullet/number
   symbol change (`gl*` `gl#` `gl-` `gl1` `gla` `gli` … plus `gL…`) has no
   nuwiki keymap; only `:…ChangeSymbol*` commands exist.