fix(mappings): add global entry-point keymaps and correct <Leader>ww/<Leader>wt targets

Two bugs prevented keymaps from working after a Dein install:

1. <Leader>ww and <Leader>wt both called diary_today() instead of
   wiki_index() / wiki_tab_index(). Fixed in keymaps.lua (Neovim)
   and ftplugin/vimwiki.vim (Vim).

2. All keymaps were buffer-local (<buffer> / buffer=bufnr), so they
   only activated inside an already-open .wiki file. Users had no way
   to reach the wiki from any other buffer, making the plugin appear
   broken on a fresh Vim start.

   Fix: register the eight <Leader>w* entry-point mappings globally —
   in setup() for Neovim (lua/nuwiki/init.lua) and in plugin/nuwiki.vim
   for plain Vim. To avoid a chicken-and-egg LSP dependency, the global
   mappings open files directly from config (wiki_root, diary_rel_path,
   file_extension); the LSP auto-starts via the FileType autocmd once
   the vimwiki buffer loads. Three new autoload helpers
   (open_wiki_path, open_diary_path, open_diary_index_path) provide the
   LSP-free path for the Vim side.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lua/nuwiki/keymaps.lua

@@ -160,8 +160,8 @@ function M.attach(bufnr, mappings)
 
   -- ===== Wiki prefix (`<Leader>w*`) =====
   if on('wiki_prefix') then
-    map('n', '<Leader>ww', cmd.diary_today,    { desc = 'nuwiki: open default wiki / today' }, bufnr)
-    map('n', '<Leader>wt', cmd.diary_today,    { desc = 'nuwiki: open today\'s diary' }, bufnr)
+    map('n', '<Leader>ww', function() cmd.wiki_index(0) end,     { desc = 'nuwiki: open wiki index' }, bufnr)
+    map('n', '<Leader>wt', function() cmd.wiki_tab_index(0) end, { desc = 'nuwiki: open wiki index (tab)' }, bufnr)
     map('n', '<Leader>ws', cmd.wiki_ui_select, { desc = 'nuwiki: pick wiki' }, bufnr)
     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)