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-mcp/src/tools/search.rs

@@ -110,6 +110,8 @@ pub fn handle(state: &KebabAppState, input: SearchInput) -> CallToolResult {
         media,
         ingested_after,
         doc_id: input.doc_id.clone().map(kebab_core::DocumentId),
+        repo: vec![],
+        code_lang: vec![],
     };
 
     let query = kebab_core::SearchQuery {