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>

doc/nuwiki.txt

@@ -433,6 +433,7 @@ Links (normal mode) ~
   <C-CR>     Follow in a vertical split.
   <C-S-CR>   Follow in a tab, reusing an existing tab if the file is
              already open.
+  <M-CR>     Add the link target to the buffer list (`:badd`, no jump).
   <BS>       Jump back (`<C-o>`).
   <Tab>      Next wikilink on or after the cursor.
   <S-Tab>    Previous wikilink.
@@ -491,7 +492,8 @@ Wiki / diary / export ~
   <Leader>wi            Open the diary index.
   <Leader>w<Leader>w    Today.
   <Leader>w<Leader>y    Yesterday.
-  <Leader>w<Leader>t    Tomorrow.
+  <Leader>w<Leader>t    Today, in a new tab.
+  <Leader>w<Leader>m    Tomorrow.
   <Leader>w<Leader>i    Rebuild the diary index page.
   <Leader>wn            Goto page (prompts for name).
   <Leader>wd            Delete the current page.