feat(rag): modularize knowledge retrieval pipeline

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

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+package com.superbiz.agent.dto;
+
+import lombok.Builder;
+import lombok.Data;
+
+import java.util.List;
+import java.util.Map;
+
+/**
+ * Normalized vector retrieval candidate before evidence post-processing.
+ */
+@Data
+@Builder
+public class RetrievedEvidenceCandidate {
+
+    private String id;
+
+    private String source;
+
+    private String title;
+
+    private String breadcrumb;
+
+    private String content;
+
+    private String retrievalLayer;
+
+    private String retrievalAttempt;
+
+    private Double score;
+
+    private Double rawScore;
+
+    private String scoreLabel;
+
+    private Integer originalRank;
+
+    private Map<String, String> metadata;
+
+    private List<String> hitReasons;
+}