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-cli/src/main.rs

@@ -828,6 +828,8 @@ fn run(cli: &Cli) -> anyhow::Result<()> {
                 media: media_norm,
                 ingested_after: ingested_after_parsed,
                 doc_id: doc_id.as_ref().map(|s| kebab_core::DocumentId(s.clone())),
+                repo: vec![],
+                code_lang: vec![],
             };
 
             let q = kebab_core::SearchQuery {