RAG reranker + manifest bucket fix — quality improvements from eval

RAG pipeline now includes a cross-encoder rerank step between retrieval
and generation. The LLM re-sorts top-K results by relevance before
they become context. Falls back to original order if model output is
unparseable (~5% with 7B models). Also improved the generation prompt
to be domain-aware ("staffing database") and request specific citations.

Fixed 4 catalog manifests with bucket="data" (pre-federation leftover)
that poisoned the entire DataFusion query context on startup. The
"users", "lab_trials", "meta_runs", and "new_candidates" datasets
now correctly reference bucket="primary". This bug was surfaced by
the quality evaluation pipeline — wouldn't have been found by
structural tests alone.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

data/_catalog/manifests/8621894c-f9c6-4eea-8e71-e80b9cdc9a36.json

@@ -0,0 +1,26 @@
+{
+  "id": "8621894c-f9c6-4eea-8e71-e80b9cdc9a36",
+  "name": "demo_customers",
+  "schema_fingerprint": "7af9708f2366f790e0ae03db6f681f5a4d4b8af4d1527b091bedfee1be81d304",
+  "objects": [
+    {
+      "bucket": "primary",
+      "key": "datasets/demo_customers.parquet",
+      "size_bytes": 4583,
+      "created_at": "2026-04-17T01:34:30.158848713Z"
+    }
+  ],
+  "created_at": "2026-04-17T01:34:30.158849587Z",
+  "updated_at": "2026-04-17T01:34:30.158849587Z",
+  "description": "",
+  "owner": "",
+  "sensitivity": null,
+  "columns": [],
+  "lineage": null,
+  "freshness": null,
+  "tags": [],
+  "row_count": null,
+  "last_embedded_at": null,
+  "embedding_stale_since": null,
+  "embedding_refresh_policy": null
+}