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/KnowledgeSearchMode.java

@@ -0,0 +1,10 @@
+package com.superbiz.agent.service.retrieval;
+
+/**
+ * Retrieval mode for {@link KnowledgeSearchPort}.
+ * Delivery 1 only requires {@link #DENSE}; hybrid arrives in a later change.
+ */
+public enum KnowledgeSearchMode {
+    DENSE,
+    HYBRID
+}