Scaffold mafiabot_core module stubs

Add assumption-free skeletons for the modules documented in the README:
empty Engine/Economy/Sockets/Exploits packages, null-body Mafiabot and
Engine_Tests procedures, and a placeholder config/config.yaml.

mafiabot_core/tests/engine_tests.adb

@@ -0,0 +1,4 @@
+procedure Engine_Tests is
+begin
+   null;
+end Engine_Tests;