Union two customer exports with different column order, unique by id, sorted numerically by id

Union two customer exports with different column order, unique by id, sorted numerically by id: a task in agent-harbor-tasks (Harbor dataset). I've got customer exports from our two shops, customers shop a.csv and customers shop b.csv. Same fields but the columns are in a different order, and some…

The task

I've got customer exports from our two shops, customers_shop_a.csv and customers_shop_b.csv. Same fields but the columns are in a different order, and some customers are in both. Combine them into all_customers.csv with columns id,name,email,country, each customer once, sorted by id (as a number).

Part of efederici/agent-harbor-tasks.