feat(rag): hybrid multi-path search with RRF fusion

Add configurable hybrid mode on KnowledgeSearchPort that fuses dense
unfiltered, dense filtered, and lexical ranks via RRF while preserving
dense-compatible threshold scores. Archives Delivery 2 OpenSpec change.

devflow/projects/2026-07-27-rag-hybrid-search-rrf/acceptance.md

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+# Acceptance: rag-hybrid-search-rrf
+
+## Result
+
+Hybrid mode implemented on KnowledgeSearchPort:
+
+- dense unfiltered + dense filtered + lexical rank over union
+- RRF fusion by evidenceKey
+- dense-compatible score preserved for thresholds
+- default mode remains dense
+
+## Verification
+
+```text
+mvn -q "-Dtest=RrfFusionTest,VectorKnowledgeSearchAdapterHybridTest,LookupKnowledgeToolTest" test
+```
+
+Pass.
+
+## Residual
+
+- True Milvus BM25/sparse schema + reindex still follow-up
+- Lexical path only ranks dense-recalled candidates (does not expand pure-term misses outside dense topK)