Make MB_MINION_SPAWN_TIME a required ConfigManager entry

0 or blank keeps the normal guard/siege minion spawn timers.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/engine/gameManager/ConfigManager.java

@@ -75,6 +75,7 @@ public enum ConfigManager {
     // Mobile AI modifiers
     MB_AI_CAST_FREQUENCY,
     MB_AI_AGGRO_RANGE,
+    MB_MINION_SPAWN_TIME,
 
     //drop rates
     MB_NORMAL_EXP_RATE,