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/src/config.rs

@@ -288,7 +288,9 @@ fn wiki_defaults() -> WikiDefaults {
         diary_rel_path: default_diary_rel_path(),
         diary_index: default_diary_index(),
         diary_frequency: default_diary_frequency(),
-        diary_caption_level: 1,
+        // Match upstream vimwiki's `diary_caption_level` default of 0 (year
+        // captions at the top level, months one below). Users override per-wiki.
+        diary_caption_level: 0,
         diary_sort: default_diary_sort(),
         diary_header: default_diary_header(),
         listsyms: default_listsyms(),