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.

src/main/java/com/superbiz/agent/service/retrieval/KnowledgeSearchRequest.java

@@ -0,0 +1,23 @@
+package com.superbiz.agent.service.retrieval;
+
+/**
+ * Portable search request used by the knowledge pipeline.
+ */
+public record KnowledgeSearchRequest(
+        String query,
+        int topK,
+        String categoryFilter,
+        KnowledgeSearchMode mode
+) {
+    public KnowledgeSearchRequest {
+        if (topK <= 0) {
+            throw new IllegalArgumentException("topK must be positive");
+        }
+        mode = mode == null ? KnowledgeSearchMode.DENSE : mode;
+        query = query == null ? "" : query;
+    }
+
+    public static KnowledgeSearchRequest dense(String query, int topK, String categoryFilter) {
+        return new KnowledgeSearchRequest(query, topK, categoryFilter, KnowledgeSearchMode.DENSE);
+    }
+}