refactor(*): Restructure game into reusable Arena/GameMode architecture with controller-driven ships, adding Free Play and Match modes

Game/scripts/goal.gd

@@ -0,0 +1,21 @@
+class_name Goal
+extends Area3D
+
+# A goal is a dumb sensor: it detects the ball crossing its plane and emits
+# goal_scored. The game mode owns all consequences (score, resets). Two of
+# these live in each arena, one per team.
+
+# The team that concedes when the ball enters this goal.
+@export var team: int = 0
+
+signal goal_scored(team: int)
+
+
+func _ready():
+	# Group lets AI controllers and game modes discover goals
+	add_to_group("goal")
+
+
+func _on_body_entered(body):
+	if body.is_in_group("ball"):
+		goal_scored.emit(team)