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>

development/tests/test-keymaps-vim.vim

@@ -728,6 +728,23 @@ endtry
 call s:record(s:nm_err ==# '' ? 1 : 0,
       \ 'cmd.VimwikiSearch_no_match_graceful', 'err=' . s:nm_err)
 
+" ===== auto_header inserts `= Stem =` on a new wiki page; skips the index =====
+let s:ah_root = fnamemodify(expand(nuwiki#commands#wiki_list()[0].root), ':p')
+if s:ah_root[len(s:ah_root)-1:] !=# '/' | let s:ah_root .= '/' | endif
+new
+execute 'file ' . fnameescape('/tmp/nuwiki-ah-plain/MyNewPage.wiki')
+call nuwiki#commands#auto_header()
+call s:record(getline(1) ==# '= MyNewPage =' ? 1 : 0,
+      \ 'cmd.auto_header_inserts', 'line1=' . getline(1))
+bwipeout!
+new
+execute 'file ' . fnameescape(s:ah_root . 'index.wiki')
+call nuwiki#commands#auto_header()
+call s:record(getline(1) ==# '' ? 1 : 0,
+      \ 'cmd.auto_header_skips_index', 'line1=' . getline(1))
+bwipeout!
+unlet s:ah_root
+
 " ===== :VimwikiGoto -nargs=* =====
 " Was -nargs=1, so a page name with a space raised E488 (and the empty-arg
 " prompt fallback was unreachable). Now -nargs=*: a multi-word arg must parse.