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>

README.md

@@ -480,6 +480,7 @@ via `mappings.<group> = false` in Neovim, or
 | `<S-CR>` | Follow in horizontal split |
 | `<C-CR>` | Follow in 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>` / `<S-Tab>` | Next / previous wikilink on or after the cursor |
 | `+` | Wrap word / visual selection as a wikilink (no follow) |
@@ -542,8 +543,9 @@ vimwiki) they fire only after `'timeoutlen'`; type a suffix such as `gln` or
 | `<Leader>wt` | Open the wiki index in a new tab |
 | `<Leader>ws` | Pick a wiki |
 | `<Leader>wi` | Open the diary index |
-| `<Leader>w<Leader>w` / `<Leader>w<Leader>y` / `<Leader>w<Leader>t` | Diary today / yesterday / tomorrow |
-| `<Leader>w<Leader>m` | Diary tomorrow (vimwiki-compat alias of `<Leader>w<Leader>t`) |
+| `<Leader>w<Leader>w` / `<Leader>w<Leader>y` | Diary today / yesterday |
+| `<Leader>w<Leader>t` | Diary today, in a new tab |
+| `<Leader>w<Leader>m` | Diary tomorrow |
 | `<Leader>w<Leader>i` | Rebuild the diary index page |
 | `<Leader>wn` / `<Leader>wd` / `<Leader>wr` | Goto / delete / rename page |
 | `<Leader>wh` / `<Leader>whh` | Export the current page to HTML / open in browser |