Add SIN.04028 tutorial material, tutor prompt and conversation log

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

SIN.04028-From-Calling-to-Eventing/supplemental-java/03-static-filesystem/Mediator.java

@@ -0,0 +1,19 @@
+void main(String[] arguments) throws Exception {
+    if (arguments.length != 2) {
+        IO.println("Usage: java Mediator.java <from-mailbox> <to-mailbox>");
+        return;
+    }
+
+    var from = arguments[0];
+    var to = arguments[1];
+
+    IO.println("Mediating: " + from + " -> " + to);
+
+    FileMessageBoard.watch(from, message -> {
+        try {
+            FileMessageBoard.publish(to, message);
+        } catch (IOException exception) {
+            throw new UncheckedIOException(exception);
+        }
+    });
+}