Create a Rillet customer from Chargebee without duplicates

**Objective:** If Chargebee customer `{chargebee-customer-id}` does not yet exist in Rillet, create it there, after the user confirms the details. Use {payment-terms} for payment terms if given.

**Step 1: Read the Chargebee customer (chargebee server)**

Call `chargebee-get-customer` with `id` = {chargebee-customer-id}. Keep `company`, `first-name`, `last-name`, `email`, `net-term-days`, `preferred-currency`. If nothing is returned, stop and say so.

**Step 2: Check Rillet for an existing customer (rillet server)**

Call `rillet-get-customer-emails` and look for the same `email` (case-insensitive). Call `rillet-get-customers` and look for an exact case-insensitive match on `name` or `invoicing-name` against the Chargebee company or full name. If any match is found, show it, do not create anything, and stop.

**Step 3: Confirm**

Show the user exactly what will be created: `name` (the company, else first plus last name), `name-on-invoice` (same), `payment-terms` ({payment-terms} if given, otherwise derived from `net-term-days` in the same wording style as existing Rillet customers' `payment-terms`), and `send-invoices-automatically` (default false). Tell the user that `rillet-add-customer` has no email field, so the email must be added in Rillet afterwards. Ask the user to confirm. Do not proceed without an explicit yes.

**Step 4: Create and verify (rillet server)**

Only after confirmation, call `rillet-add-customer` once with those values. Then call `rillet-get-customers` again and report the new `customer-id`.

**Rules & Guidelines:**

* `chargebee-get-customer` is on the chargebee server. `rillet-get-customer-emails`, `rillet-get-customers` and `rillet-add-customer` are on the rillet server.
* `rillet-add-customer` is the only write tool allowed, called at most once. Never call `rillet-update-customer`, any invoice tool, or any Chargebee tool other than `chargebee-get-customer`.

**Validation:**

* If the Chargebee customer has no company and no name, stop and ask the user for a name.
* If the verification read does not show the new customer, report the failure and do not retry.