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>

development/vimwiki-gap.md

@@ -105,8 +105,10 @@ fix site.
   `badd_link` in the links group of both clients (`lua/nuwiki/keymaps.lua`,
   `ftplugin/vimwiki.vim`; docs + README updated). Covered by `map[n].<M-CR>` in
   both keymap harnesses.
-- [ ] **`diary_caption_level` default divergence** — nuwiki `1` vs vimwiki `0`.
-  _Fix:_ `config.rs:291` (and defaults in `lua/nuwiki/config.lua`).
+- [x] **`diary_caption_level` default divergence** — nuwiki defaulted to `1`,
+  upstream `0`. _Fix:_ `wiki_defaults()` in `crates/nuwiki-lsp/src/config.rs` now
+  defaults `diary_caption_level: 0` (year captions top-level, months one below),
+  still per-wiki overridable. Test `defaults_carry_vimwiki_per_wiki_keys` updated.
 - [ ] **`diary_start_week_day`** — **wrongly removed** in `c63ec67`. The removal
   treated it as a server concern and hardwired the weekly diary to ISO-Monday
   (`crates/nuwiki-core/src/date.rs` `monday_of_iso_week`), but choosing which