feat(rag): close eval pipeline with live snapshots

devflow/projects/2026-07-06-rag-eval-pipeline-closure/brief.md

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+# Brief: rag-eval-pipeline-closure
+
+## Background
+
+The modular RAG pipeline now returns `LookupResult` with `evidenceBlocks`, `contextPack`, `retrievalTrace`, and `rerankTrace`. The RAG retrieval baseline must validate that full contract, so it can detect regressions in fallback behavior, context packing, or rerank trace.
+
+## Goals
+
+1. Reuse the existing offline RAG retrieval baseline.
+2. Extend it to support modular `LookupResult` fixtures.
+3. Add golden assertions for selected attempt, fallback reason, evidence status, context sources, and rerank top source.
+4. Add a RAG baseline diff path for regression detection.
+5. Add a snapshot generator that calls the real `LookupKnowledgeTool`.
+6. Document how RAG baseline and diagnosis baseline form a quality loop.
+
+## Non-Goals
+
+- No new production API.
+- No LLM-as-judge scoring.
+- No production API behavior changes.
+- No replacement for diagnosis eval.