feat(p10-1a-1): add SearchHit.repo / code_lang + SearchFilters.repo / code_lang

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.

crates/kebab-eval/tests/metrics_and_compare.rs

@@ -87,6 +87,8 @@ fn hit(rank: u32, chunk_id: &str, doc_id: &str) -> SearchHit {
         indexed_at: OffsetDateTime::UNIX_EPOCH,
         stale: false,
         score_kind: kebab_core::ScoreKind::Rrf,
+        repo: None,
+        code_lang: None,
     }
 }