make repair ships standby with rest of fleet if there is no one to repair (instead of advancing towards the enemy stations)

src/lib/ecs/system/ai/StandbyExecutor.h

@@ -0,0 +1,12 @@
+#pragma once
+
+class EntityAdmin;
+
+// Moves a standing-by ship toward the centroid of its other same-faction ships
+// (its fleet) so it stays among allies that may need repair, falling back to its
+// own defence stations and then to holding position when it has no allies.
+class StandbyExecutor
+{
+public:
+    void execute(EntityAdmin& admin);
+};