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>

lua/nuwiki/keymaps.lua

@@ -151,7 +151,7 @@ function M.attach(bufnr, mappings)
     map('n', '<Leader>wi', cmd.diary_index,    { desc = 'nuwiki: open diary index' }, bufnr)
     map('n', '<Leader>w<Leader>w', cmd.diary_today,     { desc = 'nuwiki: today' }, bufnr)
     map('n', '<Leader>w<Leader>y', cmd.diary_yesterday, { desc = 'nuwiki: yesterday' }, bufnr)
-    map('n', '<Leader>w<Leader>t', cmd.diary_tomorrow,  { desc = 'nuwiki: tomorrow' }, bufnr)
+    map('n', '<Leader>w<Leader>t', cmd.diary_today_tab, { desc = 'nuwiki: today in a new tab' }, bufnr)
     map('n', '<Leader>w<Leader>m', cmd.diary_tomorrow,  { desc = 'nuwiki: tomorrow (vimwiki <Leader>w<Leader>m)' }, bufnr)
     map('n', '<Leader>w<Leader>i', cmd.diary_generate_index, { desc = 'nuwiki: rebuild diary index' }, bufnr)
   end
@@ -166,6 +166,8 @@ function M.attach(bufnr, mappings)
         { desc = 'nuwiki: follow link in vsplit' }, bufnr)
     map('n', '<C-S-CR>', cmd.follow_link_drop,
         { desc = 'nuwiki: follow link in tab (reuse existing)' }, bufnr)
+    map('n', '<M-CR>',  cmd.badd_link,
+        { desc = 'nuwiki: add link target to buffer list' }, bufnr)
     map('n', '<BS>',    '<C-o>', { desc = 'nuwiki: go back', remap = true }, bufnr)
     map('n', '<Tab>',   link.next, { desc = 'nuwiki: next wikilink' }, bufnr)
     map('n', '<S-Tab>', link.prev, { desc = 'nuwiki: prev wikilink' }, bufnr)