fix(diary): default diary_caption_level to 0 (upstream parity)

nuwiki defaulted the per-wiki diary_caption_level to 1; upstream vimwiki's
default is 0 (year captions at the top level, months one below, rather than
nested one deeper). Default it to 0 in wiki_defaults(); still per-wiki
overridable. Render tests pass an explicit level so are unaffected; the
default-value assertion in defaults_carry_vimwiki_per_wiki_keys updated.

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

crates/nuwiki-lsp/tests/index_and_config.rs

@@ -267,7 +267,7 @@ fn defaults_carry_vimwiki_per_wiki_keys() {
     let cfg = WikiConfig::empty();
     assert_eq!(cfg.index, "index");
     assert_eq!(cfg.diary_frequency, "daily");
-    assert_eq!(cfg.diary_caption_level, 1);
+    assert_eq!(cfg.diary_caption_level, 0); // upstream default
     assert_eq!(cfg.diary_sort, "desc");
     assert_eq!(cfg.diary_header, "Diary");
     assert_eq!(cfg.listsyms, " .oOX");