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/src/core/engine.adb

@@ -0,0 +1,2 @@
+package body Engine is
+end Engine;

mafiabot_core/src/core/engine.ads

@@ -0,0 +1,3 @@
+pragma Elaborate_Body;
+package Engine is
+end Engine;