Add missing :Nuwiki* aliases and drop dead deferred-stub code

Every :Vimwiki* command now has a canonical :Nuwiki* counterpart
(tables, colorize, clipboard paste, list-level/remove-done, and the
Neovim-only split/tab link-follow variants). Remove the now-unused
"not yet implemented" stub helpers (_not_yet, deferred,
s:notify_deferred) and the stale "deferred" comments, since every
command is implemented and server-backed.

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

lua/nuwiki/commands.lua

@@ -468,21 +468,6 @@ function M.rename_file()
   vim.lsp.buf.rename()
 end
 
--- ===== Deferred placeholders =====
---
--- These commands aren't implemented on the server yet. Stub them so the
--- `:Vimwiki*` compat surface exists and users get a clear message
--- instead of "unknown command".
-
-local function _not_yet(name)
-  return function()
-    vim.notify(
-      'nuwiki: ' .. name .. ' is not yet implemented',
-      vim.log.levels.WARN
-    )
-  end
-end
-
 -- Cluster A — list rewriters.
 
 -- `gl<Space>` — remove done items from the current list only. Passing the