feat(config): on-save autoregen family — links/tags/diary-index (P2)

Added per-wiki auto_generate_links, auto_generate_tags, auto_diary_index
(default false, like upstream), wired into the did_save hook mirroring
auto_toc:

- auto_generate_links / auto_generate_tags rebuild the Generated Links /
  Generated Tags section ONLY when it already exists (new
  links_rebuild_edit / tag_links_rebuild_edit, like toc_rebuild_edit) — never
  inserts one into a page that lacks it.
- auto_diary_index regenerates the diary index page when a dated diary entry
  (not the index itself) is saved, via diary_generate_index_edit (handles the
  index file open / on-disk / absent).

auto_tags is recorded as an intentional divergence: nuwiki re-indexes tags on
every change, so the tag metadata is always fresh (no on-disk file to update).

Tests: links_rebuild_edit_only_acts_when_section_present,
tag_links_rebuild_edit_only_acts_when_index_present, config round-trip +
defaults. README/doc/lua comment updated. fmt/clippy clean; 0 Rust failures.

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

lua/nuwiki/config.lua

@@ -25,6 +25,7 @@ M.defaults = {
   --   diary_caption_level, diary_sort, diary_header
   --   html_path, template_path, template_default, template_ext,
   --   template_date_format, css_name, auto_export, auto_toc,
+  --   auto_generate_links, auto_generate_tags, auto_diary_index,
   --   html_filename_parameterization, exclude_files, color_dic
   --   toc_header, toc_header_level, links_header, links_header_level,
   --   tags_header, tags_header_level,