feat(calendar): integrate calendar-vim with wiki diary paths

Wire g:calendar_action/g:calendar_sign to nuwiki diary hooks that
resolve diary paths from the wiki root config instead of calendar-vim's
default ~/diary. Auto-detected on FileType vimwiki when calendar-vim is
present; opt out with g:nuwiki_use_calendar=0 or g:nuwiki_no_calendar.

- autoload/nuwiki/diary.vim: calendar_action/calendar_sign using wiki cfg;
  open diary in the previous window and close the calendar window after.
- autoload/vimwiki/diary.vim: vimwiki# aliases for drop-in compatibility.
- plugin/nuwiki.vim + lua/nuwiki/init.lua: auto-wire hooks, overwriting
  calendar-vim's defaults while respecting user-set custom hooks.
- lua/nuwiki/config.lua: add use_calendar option (on by default).
- development/start-*.sh + _common.sh: clone calendar-vim, fix vimrc var
  expansion, pass wiki root to the Vim/Neovim clients.

plugin/nuwiki.vim

@@ -27,6 +27,30 @@ else
   command! -nargs=0 NuwikiInstall execute 'source' fnameescape(s:plugin_root . '/scripts/download_bin.vim')
 endif
 
+" Calendar-Vim Integration — auto-detect when calendar-vim is loaded.
+augroup nuwiki_calendar_detect
+  autocmd!
+  autocmd FileType vimwiki call s:nuwiki_setup_calendar()
+augroup END
+function! s:nuwiki_setup_calendar() abort
+  if !get(g:, 'nuwiki_use_calendar', 1) || get(g:, 'nuwiki_no_calendar', 0)
+    return
+  endif
+  if exists('*calendar#open') || exists(':Calendar') == 2
+    " Wire nuwiki's diary hooks unless the user set a custom (non-default) one.
+    " calendar-vim's own default is 'calendar#diary' (writes to ~/diary); we
+    " overwrite that so paths come from the wiki root config instead.
+    let l:act = get(g:, 'calendar_action', '')
+    if l:act ==# '' || l:act ==# 'calendar#diary'
+      let g:calendar_action = 'vimwiki#diary#calendar_action'
+    endif
+    let l:sign = get(g:, 'calendar_sign', '')
+    if l:sign ==# '' || l:sign ==# 'calendar#sign'
+      let g:calendar_sign = 'vimwiki#diary#calendar_sign'
+    endif
+  endif
+endfunction
+
 if has('nvim')
   " Neovim path is initialised by `require('nuwiki').setup()`. Nothing else
   " to do at vimscript boot — wait until the user runs setup.