feat(config): P3 config batch — group 2 (create_link, dir_link, auto_header)

- create_link (default true): server resolve_target_uri now only synthesises a
  missing-page URI when create_link is set; when off, following a link to a
  non-existent page returns no location (no-op).
- dir_link: a [[dir/]] link now resolves to <dir>/<dir_link><ext> when dir_link
  is set, else opens the directory itself. Uses the already-parsed
  LinkTarget.is_directory.
- auto_header (default off): new wiki pages get a `= Stem =` header derived from
  the filename, skipping the wiki index + diary index. Client-side BufNewFile
  autocmd in both clients (init.lua + plugin/nuwiki.vim), with auto_header
  handlers in both command layers (trailing-slash-robust root matching).

Tests: cmd.auto_header_inserts + cmd.auto_header_skips_diary_index (nvim) /
cmd.auto_header_skips_index (vim). Full rust suite + both harnesses green
(Neovim 307, Vim 301/18/21).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plugin/nuwiki.vim

@@ -69,6 +69,19 @@ augroup nuwiki_vim_lsp_start
   autocmd FileType vimwiki call nuwiki#lsp#start()
 augroup END
 
+" vimwiki `auto_header` (default off): insert a level-1 header from the filename
+" on new wiki pages. BufNewFile fires only for genuinely new files. Matches the
+" configured extension(s).
+if get(g:, 'nuwiki_auto_header', 0)
+  let s:awh_ext = get(g:, 'nuwiki_file_extension', '.wiki')
+  let s:awh_ext = s:awh_ext[0] ==# '.' ? s:awh_ext : '.' . s:awh_ext
+  augroup nuwiki_auto_header
+    autocmd!
+    execute 'autocmd BufNewFile *' . s:awh_ext . ' call nuwiki#commands#auto_header()'
+  augroup END
+  unlet s:awh_ext
+endif
+
 " Global wiki-picker commands — available from any buffer so a wiki can be
 " chosen before a .wiki file exists. The picker reads its list from config
 " (no LSP); opening the chosen index then auto-starts the server. The