docs(calendar): fix stale comments and integration docs

- diary.vim: correct calendar_action doc comment (dir is used for split
  direction; only week is unused)
- calendar-vim-integration.md: use real nuwiki var names
  (g:nuwiki_use_calendar / g:nuwiki_no_calendar / g:nuwiki_wikis), document
  auto-detect wiring and calendar-window close behavior; drop the
  non-existent refresh-on-close autocmd
- vimwiki-gap.md: remove stale 'no calendar.vim integration' entry now
  that the integration is implemented

development/vimwiki-gap.md

@@ -179,7 +179,6 @@ audits don't re-flag them.
   backed by LSP foldingRange.
 - `key_mappings` (dict) — replaced by Lua `mappings.<group>` + the
   `g:nuwiki_no_<group>_mappings` globals.
-- `use_calendar` — no calendar.vim integration.
 - `CJK_length`, `listing_hl*`, `schemes_*`, `w32_dir_enc`, `menu`,
   `rx_todo` / `tag_format` — syntax internals, menu, or Vim/Win shims.