Wire two new optional fields onto SearchHit (skip_serializing_if = None)
and two Vec<String> filter fields onto SearchFilters (serde default).
Add RetrievalDetail::Default impl (manual, uses SearchMode::Hybrid as
sentinel). Patch all downstream SearchHit / SearchFilters literal
constructors with repo: None / code_lang: None / vec![] as appropriate.
Also covers Citation::Code arm in kebab-eval metrics match.
fa4eeb5a87