feat(config): P3 config batch — group 1 (clean config + plumbing)

Config plumbing for the whole config batch (HtmlConfig + WikiConfig fields,
RawWiki deserialization, From/defaults) plus these behaviors:

- rss_name / rss_max_items: HtmlConfig fields (wired into write_rss in the RSS
  group).
- toc_link_format: build_toc_text emits [[#anchor]] (no description) for 1,
  [[#anchor|title]] for 0.
- generated_links_caption: build_links_text emits [[page|FirstHeading]] from a
  page->heading map (ops::page_captions) when enabled.
- table_reduce_last_col: column_widths clamps the last column to width 1 when
  set; threaded through table_align_edit/table_move_column_edit + commands.
- color_dic now ships a populated default palette (red/green/blue/...).
- commentstring aligned to upstream's no-space `%%%s`.
- auto_chdir (default off): :lcd into the owning wiki root on buffer enter;
  both clients (ftplugin.lua setup_auto_chdir + Vim NuwikiAutoChdir augroup),
  with config.wiki_root_for / nuwiki#commands#wiki_root_for resolvers.

Also seeded HtmlConfig fields for later groups (valid_html_tags,
list/text_ignore_newline, emoji_enable, user_htmls, color_tag_template) and
WikiConfig (create_link, dir_link, bullet_types, cycle_bullets).

Tests: toc_link_format, generated_links_caption, table_reduce_last_col,
config defaults + JSON round-trip for every new key. Full rust suite +
both keymap harnesses green.

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

autoload/nuwiki/commands.vim

@@ -168,6 +168,28 @@ endfunction
 " from the server, so the picker works from any buffer before a wiki file is
 " ever opened — opening the chosen index then auto-starts the LSP via the
 " FileType autocmd. Plain Vim has no `vim.ui.select`, so use `inputlist()`.
+" Absolute root (with trailing slash) of the wiki that owns `a:file`, or the
+" first configured wiki as a fallback. Used by auto_chdir + search scoping.
+function! nuwiki#commands#wiki_root_for(file) abort
+  for l:w in nuwiki#commands#wiki_list()
+    let l:wroot = fnamemodify(expand(l:w.root), ':p')
+    if a:file[: len(l:wroot) - 1] ==# l:wroot
+      return l:wroot
+    endif
+  endfor
+  let l:wikis = nuwiki#commands#wiki_list()
+  return empty(l:wikis) ? '' : fnamemodify(expand(l:wikis[0].root), ':p')
+endfunction
+
+" :NuwikiAutoChdir hook — `:lcd` into the owning wiki's root (vimwiki
+" `auto_chdir`). Wired from a buffer-local autocmd in the ftplugin.
+function! nuwiki#commands#auto_chdir() abort
+  let l:root = nuwiki#commands#wiki_root_for(expand('%:p'))
+  if l:root !=# ''
+    execute 'lcd ' . fnameescape(l:root)
+  endif
+endfunction
+
 " :VimwikiShowVersion / :NuwikiShowVersion — echo the nuwiki version and the
 " host editor, mirroring upstream's version banner.
 function! nuwiki#commands#show_version() abort