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.

autoload/nuwiki/diary.vim

@@ -0,0 +1,97 @@
+" autoload/nuwiki/diary.vim — diary-specific functions for nuwiki
+" Includes calendar-vim integration functions
+
+" ===== Calendar-Vim Integration =====
+
+" Function: nuwiki#diary#calendar_action(day, month, year, week, dir)
+" Called by calendar-vim when user presses Enter on a date.
+" day/month/year are numeric integers. week and dir are not used.
+" Opens or creates the diary entry for the given date.
+function! nuwiki#diary#calendar_action(day, month, year, week, dir) abort
+  " Build YYYY-MM-DD from the parts passed by calendar-vim
+  let l:date_str = printf('%04d-%02d-%02d', a:year, a:month, a:day)
+
+  " Get wiki configuration for the first wiki (index 0)
+  let l:c = s:wiki_cfg(0)
+
+  " Construct file path: {root}/{diary_rel}/{YYYY-MM-DD}.{ext}
+  let l:file_path = l:c.root . '/' . l:c.diary_rel . '/' . l:date_str . l:c.ext
+
+  " Remember the calendar window so we can close it once the diary opens.
+  let l:cal_winid = win_getid()
+
+  " Open the diary file in a different window. If a previous window exists go
+  " there; otherwise split / vsplit / new depending on a:dir.
+  if winnr('#') > 0
+    execute 'wincmd p'
+    if !&hidden && &modified
+      execute 'new'
+    endif
+  elseif a:dir ==# 'V'
+    execute 'vsplit'
+  else
+    execute 'split'
+  endif
+
+  execute 'edit ' . fnameescape(l:file_path)
+  if !filereadable(l:file_path)
+    if l:c.ext[0] ==# '.'
+      execute 'setfiletype ' . l:c.ext[1:]
+    else
+      execute 'setfiletype ' . l:c.ext
+    endif
+  endif
+
+  " Close the calendar window now that the diary is open (keep at least one).
+  if win_getid() != l:cal_winid && win_id2win(l:cal_winid) > 0 && winnr('$') > 1
+    let l:diary_winid = win_getid()
+    if win_gotoid(l:cal_winid)
+      close
+    endif
+    call win_gotoid(l:diary_winid)
+  endif
+endfunction
+
+" Function: nuwiki#diary#calendar_sign(day, month, year)
+" Called by calendar-vim to determine which dates show markers (*)
+" day, month, year are numeric integers (1-based).
+" Returns a single-character string to use as marker (e.g. '*' or ''),
+" or 1/0 for 1-indexed calendars.
+" calendar-vim's default expects the return value to be a single character
+" to display; returning any non-empty string shows a marker.
+function! nuwiki#diary#calendar_sign(day, month, year) abort
+  " Build YYYY-MM-DD from the parts passed by calendar-vim
+  let l:date_str = printf('%04d-%02d-%02d', a:year, a:month, a:day)
+
+  " Get wiki configuration for the first wiki (index 0)
+  let l:c = s:wiki_cfg(0)
+
+  " Construct expected file path
+  let l:file_path = l:c.root . '/' . l:c.diary_rel . '/' . l:date_str . l:c.ext
+
+  " Return a marker if the file exists, empty string otherwise
+  if filereadable(l:file_path)
+    return '*'
+  endif
+  return ''
+endfunction
+
+" Helper: get wiki configuration for wiki n (0-based).
+" Mirrors the same logic used by nuwiki#commands#open_diary_path etc.
+function! s:wiki_cfg(n) abort
+  let l:w = {}
+  if exists('g:nuwiki_wikis') && len(g:nuwiki_wikis) > a:n
+    let l:w = g:nuwiki_wikis[a:n]
+  endif
+  let l:root  = expand(get(l:w, 'root',
+        \ get(g:, 'nuwiki_wiki_root', '~/vimwiki')))
+  let l:ext   = get(l:w, 'file_extension',
+        \ get(g:, 'nuwiki_file_extension', '.wiki'))
+  if l:ext[0] !=# '.' | let l:ext = '.' . l:ext | endif
+  let l:idx       = get(l:w, 'index', 'index')
+  let l:diary_rel = get(l:w, 'diary_rel_path',
+        \ get(g:, 'nuwiki_diary_rel_path', 'diary'))
+  let l:diary_idx = get(l:w, 'diary_index', 'diary')
+  return { 'root': l:root, 'ext': l:ext, 'idx': l:idx,
+        \  'diary_rel': l:diary_rel, 'diary_idx': l:diary_idx }
+endfunction