Add all 23 GoF design pattern implementations

01-creational/factory-method/SlackNotification.java

@@ -0,0 +1,11 @@
+package factorymethod;
+
+// New Concrete Product — knows how to send to Slack
+public class SlackNotification implements Notification {
+    @Override
+    public void send(String recipient, String message) {
+        System.out.printf("  [SLACK] Channel: %s | Message: %s%n", recipient, message);
+    }
+    @Override
+    public String getType() { return "SLACK"; }
+}