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>

autoload/nuwiki/commands.vim

@@ -190,6 +190,30 @@ function! nuwiki#commands#auto_chdir() abort
   endif
 endfunction
 
+" vimwiki `auto_header`: on a new wiki page, insert a level-1 header derived
+" from the filename. Skips the wiki index + diary index pages; only acts on an
+" empty new buffer. Wired from a BufNewFile autocmd.
+function! nuwiki#commands#auto_header() abort
+  if line('$') > 1 || getline(1) !=# ''
+    return
+  endif
+  let l:file = expand('%:p')
+  for l:c in nuwiki#commands#wiki_list()
+    let l:root = fnamemodify(expand(l:c.root), ':p')
+    if l:root[len(l:root) - 1:] !=# '/' | let l:root .= '/' | endif
+    if l:file[: len(l:root) - 1] ==# l:root
+      let l:index_path = l:root . l:c.idx . l:c.ext
+      let l:diary_index_path = l:root . l:c.diary_rel . '/' . l:c.diary_idx . l:c.ext
+      if l:file ==# l:index_path || l:file ==# l:diary_index_path
+        return
+      endif
+      break
+    endif
+  endfor
+  call setline(1, '= ' . expand('%:t:r') . ' =')
+  call append(1, '')
+endfunction
+
 " :VimwikiShowVersion / :NuwikiShowVersion — echo the nuwiki version and the
 " host editor, mirroring upstream's version banner.
 function! nuwiki#commands#show_version() abort