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>

ftplugin/vimwiki.vim

@@ -206,7 +206,7 @@ if !has('nvim')
       nnoremap <silent><buffer> <Leader>wi  :call nuwiki#commands#diary_index()<CR>
       nnoremap <silent><buffer> <Leader>w<Leader>w  :call nuwiki#commands#diary_today()<CR>
       nnoremap <silent><buffer> <Leader>w<Leader>y  :call nuwiki#commands#diary_yesterday()<CR>
-      nnoremap <silent><buffer> <Leader>w<Leader>t  :call nuwiki#commands#diary_tomorrow()<CR>
+      nnoremap <silent><buffer> <Leader>w<Leader>t  :call nuwiki#commands#diary_today_tab()<CR>
       nnoremap <silent><buffer> <Leader>w<Leader>m  :call nuwiki#commands#diary_tomorrow()<CR>
       nnoremap <silent><buffer> <Leader>w<Leader>i  :call nuwiki#commands#diary_generate_index()<CR>
     endif
@@ -217,6 +217,7 @@ if !has('nvim')
       nnoremap <silent><buffer> <S-CR>     :split<CR>:call nuwiki#commands#follow_link_or_create()<CR>
       nnoremap <silent><buffer> <C-CR>     :vsplit<CR>:call nuwiki#commands#follow_link_or_create()<CR>
       nnoremap <silent><buffer> <C-S-CR>   :call nuwiki#commands#follow_link_drop()<CR>
+      nnoremap <silent><buffer> <M-CR>     :call nuwiki#commands#badd_link()<CR>
       nnoremap <silent><buffer> <BS>       <C-o>
       nnoremap <silent><buffer> <Tab>      /\[\[<CR>:nohlsearch<CR>
       nnoremap <silent><buffer> <S-Tab>    ?\[\[<CR>:nohlsearch<CR>