Add all 23 GoF design pattern implementations (2026-06-13)

03-behavioral/mediator/ChatMediator.java

@@ -0,0 +1,7 @@
+package mediator;
+
+/** Mediator interface — defines how colleagues communicate through the hub */
+public interface ChatMediator {
+    void sendMessage(String message, User sender);
+    void addUser(User user);
+}