fix(dogfood): populate schema.v1.repo_breakdown (Task 9 follow-up)

Dogfooding (PR #142 1B + multi-root corpus: kebab-docs + httpx + zod + lodash)
revealed schema.v1.repo_breakdown is always {} despite the 1A-2 Task 9
having added the code_lang_breakdown sibling. The schema.rs:171 placeholder
`BTreeMap::new()` was left in place. Mirror Task 9's code_lang_breakdown
query for the repo field — same metadata_json JSON-path pattern.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/kebab-app/src/schema.rs

@@ -168,7 +168,9 @@ fn collect_stats(
         stale_doc_count: counts.stale_doc_count,
         // p10-1A-2: populated by the store query added in this task.
         code_lang_breakdown: store.code_lang_breakdown()?,
-        repo_breakdown: std::collections::BTreeMap::new(),
+        // p10-1A-2 follow-up: dogfooding (2026-05-20) revealed this was a
+        // placeholder — mirror of code_lang_breakdown for the repo field.
+        repo_breakdown: store.repo_breakdown()?,
     })
 }