feat(rag): chunk evidence identity, dedup, and search port

Preserve same-document multi-chunk evidence with evidenceKey identity,
per-document caps, retrieve-k/return-n split, and a dense KnowledgeSearchPort.
Archives Delivery 1 OpenSpec change as the foundation for hybrid retrieval.

openspec/specs/rag-knowledge-retrieval/spec.md

@@ -54,14 +54,23 @@ The `lookup_knowledge` retrieval flow SHALL expose retrieved evidence as structu
 - **AND** the result SHALL NOT rely on legacy `primary` or `supplement` fields for L0/L1 meaning
 
 ### Requirement: Knowledge retrieval SHALL deduplicate evidence blocks
-The retrieval flow SHALL remove duplicate evidence blocks before returning them to the Agent.
 
-#### Scenario: Duplicate source deduplication
-- **WHEN** L0 and L1 produce evidence with the same source identity
-- **THEN** the retrieval flow SHALL keep a single evidence block for that source
-- **AND** the evidence block SHALL preserve hit reasons from both retrieval paths when available
+The retrieval flow SHALL remove duplicate evidence blocks before returning them to the Agent. Duplicates are defined by chunk-level `evidenceKey` identity, not by document source alone.
+
+#### Scenario: Chunk-level deduplication keeps distinct chunks
+
+- **WHEN** L1 produces multiple candidates with the same source but different chunk identities
+- **THEN** the retrieval flow SHALL keep separate evidence blocks for those chunk identities subject to per-document caps
+- **AND** SHALL NOT collapse them solely because source is equal
+
+#### Scenario: Same evidenceKey collapses
+
+- **WHEN** two candidates share the same evidenceKey
+- **THEN** the retrieval flow SHALL keep a single evidence block for that identity
+- **AND** the evidence block SHALL preserve hit reasons from both paths when available
 
 #### Scenario: Postprocess count tracking
+
 - **WHEN** evidence post-processing completes
 - **THEN** the tool invocation details SHALL record candidate count and final evidence block count
 
@@ -248,3 +257,4 @@ The `lookup_knowledge` tool SHALL persist modular RAG pipeline details in `tool_
 - **WHEN** `lookup_knowledge` returns no usable evidence
 - **THEN** `retrieval_details` SHALL still include retrieval trace information for attempted retrieval paths
 - **AND** it SHALL not include full evidence content as duplicated trace data
+