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.

development/_common.sh

@@ -275,6 +275,20 @@ reset_dirs() {
   done
 }
 
+# ensure_calendar_vim — clone calendar-vim into dev dir for testing
+ensure_calendar_vim() {
+  local repo="$DEV_DIR/calendar-vim"
+  if [[ -d "$repo/.git" ]]; then
+    return
+  fi
+  if ! command -v git >/dev/null 2>&1; then
+    log "git not found — skipping calendar-vim"
+    return 1
+  fi
+  log "cloning https://github.com/mattn/calendar-vim.git → $repo"
+  git clone --depth 1 https://github.com/mattn/calendar-vim.git "$repo" >/dev/null 2>&1
+}
+
 seed_wiki() {
   if [[ "${NUWIKI_DEV_NO_SEED:-0}" == "1" ]]; then
     log "NUWIKI_DEV_NO_SEED=1 — skipping wiki seed (using $WIKI_DIR as-is)"