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-search/src/hybrid.rs

@@ -509,6 +509,8 @@ mod tests {
             indexed_at: time::OffsetDateTime::UNIX_EPOCH,
             stale: false,
             score_kind: kebab_core::ScoreKind::Rrf,
+            repo: None,
+            code_lang: None,
         }
     }
 
@@ -760,6 +762,8 @@ mod tests {
                 indexed_at: time::OffsetDateTime::UNIX_EPOCH,
                 stale: false,
                 score_kind: kebab_core::ScoreKind::Rrf,
+                repo: None,
+                code_lang: None,
             }
         }
 

crates/kebab-search/src/lexical.rs

@@ -470,6 +470,8 @@ fn build_hit(
         // in `RagPipeline::ask` against the configured threshold.
         stale: false,
         score_kind: ScoreKind::Bm25,
+        repo: None,
+        code_lang: None,
     })
 }
 

crates/kebab-search/src/vector.rs

@@ -327,6 +327,8 @@ fn build_hit(
         // in `RagPipeline::ask` against the configured threshold.
         stale: false,
         score_kind: ScoreKind::Cosine,
+        repo: None,
+        code_lang: None,
     })
 }