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.lua

@@ -305,6 +305,31 @@ vim.defer_fn(function()
     vim.cmd('bwipeout!')
   end
 
+  -- auto_header: the function inserts `= Stem =` on an empty new wiki buffer,
+  -- and skips the wiki index page. Operate on dedicated buffer handles so the
+  -- harness's own buffer is untouched.
+  do
+    local wroot = require('nuwiki.config').wiki_cfg(0).root
+    local root = vim.fn.fnamemodify(vim.fn.expand(wroot), ':p')
+    if root:sub(-1) ~= '/' then root = root .. '/' end
+    local function ah_line(name)
+      local buf = vim.api.nvim_create_buf(true, false)
+      vim.api.nvim_buf_set_name(buf, name)
+      require('nuwiki.commands').auto_header(buf)
+      local line = vim.api.nvim_buf_get_lines(buf, 0, 1, false)[1] or ''
+      vim.api.nvim_buf_delete(buf, { force = true })
+      return line
+    end
+    -- A normal page (outside any wiki root) → header inserted.
+    local got = ah_line('/tmp/nuwiki-ah-plain/MyNewPage.wiki')
+    record(got == '= MyNewPage =', 'cmd.auto_header_inserts', 'line1=' .. got)
+    -- The diary index page → skipped (no header). (The wiki index itself is the
+    -- harness's open buffer, so use the diary index to avoid an E95 collision.)
+    local c = require('nuwiki.config').wiki_cfg(0)
+    local idx_line = ah_line(root .. c.diary_rel .. '/' .. c.diary_idx .. '.wiki')
+    record(idx_line == '', 'cmd.auto_header_skips_diary_index', 'line1=' .. idx_line)
+  end
+
   -- ===== Lists =====
 
   run('list.toggle_via_C-Space', {