feat(rag): modularize knowledge retrieval pipeline

src/main/java/com/superbiz/agent/dto/EvidencePostprocessResult.java

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+package com.superbiz.agent.dto;
+
+import lombok.Builder;
+import lombok.Data;
+
+import java.util.List;
+
+/**
+ * Output of post-retrieval processing before context packing.
+ */
+@Data
+@Builder
+public class EvidencePostprocessResult {
+
+    private Integer candidateCount;
+
+    private Integer evidenceBlockCount;
+
+    private List<EvidenceBlock> evidenceBlocks;
+
+    private String relevanceLevel;
+
+    private String completenessHint;
+
+    private RerankTrace rerankTrace;
+
+    private Double topSimilarity;
+
+    public boolean hasUsableEvidence() {
+        return evidenceBlocks != null && !evidenceBlocks.isEmpty();
+    }
+}