fix(client): P2 diary tab collision + <M-CR> badd-link parity

Two upstream-parity keymap fixes (confirmed against upstream
ftplugin/vimwiki.vim):

1. <Leader>w<Leader>t collision — nuwiki bound both <Leader>w<Leader>t and
   <Leader>w<Leader>m to tomorrow's diary. Upstream: <Leader>w<Leader>t opens
   today's diary in a NEW TAB, <Leader>w<Leader>m opens tomorrow. <Leader>w
   <Leader>t now calls diary_today_tab; <Leader>w<Leader>m stays tomorrow.

2. <M-CR> badd-link — adding a link target to the buffer list was mouse-only
   (<MiddleMouse>) / command-only (:VimwikiBaddLink). Upstream binds
   <M-CR> -> VimwikiBaddLink. Added normal-mode <M-CR> -> badd_link to the
   links group of both clients.

Both clients (lua/nuwiki/keymaps.lua, ftplugin/vimwiki.vim); README + doc/
nuwiki.txt updated. Tests: map[n].<M-CR> in both harnesses;
diary.leader_t_opens_today_in_tab / diary.leader_m_opens_tomorrow (vim).
nvim 270, vim 263+18, all green.

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

development/tests/test-keymaps.lua

@@ -206,6 +206,7 @@ vim.defer_fn(function()
   local mapping_surface = {
     -- Links
     { '<CR>', 'n' }, { '<S-CR>', 'n' }, { '<C-CR>', 'n' }, { '<C-S-CR>', 'n' },
+    { '<M-CR>', 'n' },
     { '<BS>', 'n' }, { '<Tab>', 'n' }, { '<S-Tab>', 'n' }, { '+', 'nx' },
     -- Lists
     { '<C-Space>', 'nx' }, { '<C-@>', 'nx' }, { '<Nul>', 'nx' },