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>

crates/nuwiki-lsp/tests/link_health.rs

@@ -305,6 +305,39 @@ fn build_toc_text_with_no_headings_is_just_the_heading() {
     assert_eq!(out, "= Contents =\n");
 }
 
+#[test]
+fn links_rebuild_edit_only_acts_when_section_present() {
+    let pages = vec!["Home".to_string(), "About".to_string()];
+    let uri = Url::from_file_path("/w/Home.wiki").unwrap();
+    let without = "= Home =\nbody\n";
+    assert!(
+        ops::links_rebuild_edit(
+            without,
+            &parse(without),
+            &uri,
+            "Home",
+            &pages,
+            true,
+            "Generated Links",
+            1
+        )
+        .is_none(),
+        "must not insert a links section into a page that lacks one"
+    );
+    let with = "= Generated Links =\n- [[old]]\n";
+    assert!(ops::links_rebuild_edit(
+        with,
+        &parse(with),
+        &uri,
+        "Home",
+        &pages,
+        true,
+        "Generated Links",
+        1
+    )
+    .is_some());
+}
+
 #[test]
 fn find_section_range_stops_at_same_level_sibling() {
     // A non-level-1 generated section (e.g. a level-2 tags index) must not