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>

lua/nuwiki/commands.lua

@@ -285,6 +285,37 @@ function M.show_version()
   }, false, {})
 end
 
+-- vimwiki `auto_header`: on a new wiki page, insert a level-1 header derived
+-- from the filename. Skips the wiki index + diary index pages, and only acts
+-- on a genuinely empty buffer. Called from a BufNewFile autocmd.
+function M.auto_header(bufnr)
+  bufnr = bufnr or vim.api.nvim_get_current_buf()
+  local lines = vim.api.nvim_buf_get_lines(bufnr, 0, -1, false)
+  if #lines > 1 or (lines[1] or '') ~= '' then
+    return -- not an empty new buffer
+  end
+  local file = vim.api.nvim_buf_get_name(bufnr)
+  local config = require('nuwiki.config')
+  for i = 0, 64 do
+    local c = config.wiki_cfg(i)
+    if not c then break end
+    local root = vim.fn.fnamemodify(vim.fn.expand(c.root), ':p')
+    if root:sub(-1) ~= '/' then root = root .. '/' end
+    if file:sub(1, #root) == root then
+      -- Skip the wiki index and the diary index pages.
+      local index_path = root .. c.idx .. c.ext
+      local diary_index_path = root .. c.diary_rel .. '/' .. c.diary_idx .. c.ext
+      if file == index_path or file == diary_index_path then
+        return
+      end
+      break
+    end
+    if not (config.options.wikis or vim.g.nuwiki_wikis) then break end
+  end
+  local title = vim.fn.fnamemodify(file, ':t:r')
+  vim.api.nvim_buf_set_lines(bufnr, 0, 1, false, { '= ' .. title .. ' =', '' })
+end
+
 -- :VimwikiSearch / :VWS {pattern} — search the current wiki's files (upstream
 -- scopes to the wiki, not the editor CWD). Resolves the wiki whose root holds
 -- the current file, else the first configured wiki, else CWD. Empty pattern