Add shop sell-back

scripts/core/campaign_state.gd

@@ -256,6 +256,25 @@ func try_buy_item(item_id: String, price: int) -> bool:
 	return false
 
 
+func try_sell_item(item_id: String, sale_price: int) -> bool:
+	if item_id.is_empty() or sale_price <= 0:
+		return false
+	var previous_count := int(inventory.get(item_id, 0))
+	if previous_count <= 0:
+		return false
+	var previous_gold := gold
+	gold += sale_price
+	if previous_count > 1:
+		inventory[item_id] = previous_count - 1
+	else:
+		inventory.erase(item_id)
+	if save_game():
+		return true
+	gold = previous_gold
+	inventory[item_id] = previous_count
+	return false
+
+
 func try_save_prebattle_loadout(roster_snapshot: Dictionary, inventory_snapshot: Dictionary) -> bool:
 	var previous_roster := roster.duplicate(true)
 	var previous_inventory := inventory.duplicate(true)