test(calendar): add Vim + Neovim integration harnesses

Add headless harnesses covering the calendar-vim integration, wired into
CI alongside the existing keymap/config suites:

- test-calendar-vim.sh/.vim (+ -optout.vim): pure-VimL diary hooks
  (calendar_sign markers, calendar_action path resolution from wiki root,
  calendar window close), plugin/nuwiki.vim hook auto-wiring that
  overrides calendar-vim's defaults, and opt-out via g:nuwiki_use_calendar
  and g:nuwiki_no_calendar. Uses a stub calendar-vim on the runtimepath.
- test-calendar.sh/.lua: Neovim setup() wiring + window-close under the
  real window API, and opt-out via use_calendar = false.

Fixes surfaced by the suite:
- diary.vim: new diary entries set filetype 'vimwiki' (was the raw
  extension, e.g. 'wiki'); the stray FileType autocmd also blocked the
  calendar window from closing.
- init.lua: drop the redundant Neovim FileType autocmd (plugin/nuwiki.vim
  already wires both editors) and translate use_calendar=false into
  g:nuwiki_no_calendar so the opt-out actually disables wiring.

development/ONBOARDING.md

@@ -79,7 +79,12 @@ nuwiki/
 │       ├── test-keymaps.sh     # Neovim keymap harness
 │       ├── test-keymaps.lua
 │       ├── test-keymaps-vim.sh # Vim keymap harness
-│       └── test-keymaps-vim.vim
+│       ├── test-keymaps-vim.vim
+│       ├── test-calendar.sh    # Neovim calendar-vim integration harness
+│       ├── test-calendar.lua
+│       ├── test-calendar-vim.sh # Vim calendar-vim integration harness
+│       ├── test-calendar-vim.vim
+│       └── test-calendar-vim-optout.vim
 │
 └── .gitea/
     └── workflows/