Add Rillet customers Drip does not have yet

**Objective:** Create a Drip subscriber, tagged `{tag}` (default Customer), for every Rillet customer email of type `{email-type}` (default: every type) that Drip does not have yet.

**Step 1: Read Rillet customer emails**

Call `rillet-get-customer-emails` (Rillet) once. Filter to `{email-type}` if set. Drop blank and malformed emails, and de-duplicate by lower-cased email.

**Step 2: Find the gaps in Drip**

For each email, call `drip-get-subscriber` (Drip) with email set to it. Keep only emails with no Drip record at all. An email that exists in Drip with status unsubscribed is not a gap: leave it alone.

**Step 3: Confirm**

Show the list to the user: email, customer name, country, and the tag that will be applied. Ask for explicit confirmation before writing anything. If they remove rows, drop them.

**Step 4: Write**

For each confirmed email, call `drip-add-subscriber` (Drip) once with email, tags set to `{tag}` (or Customer), and country from Rillet when present. Leave status, eu_consent and prospect unset. If the customer name splits cleanly into a person's first and last name, set first_name and last_name; if it is a company name, leave them blank.

**Step 5: Confirm the result**

Call `drip-get-subscriber` (Drip) for each added email and report which now exist, and any that failed with the error returned.

**Rules & Guidelines:**

* The only key shared by the two systems is the email address. Rillet emails come from `rillet-get-customer-emails` (one row per address, with its customer-id and email-type); Drip emails come from the email field of `drip-get-subscriber`. Compare them trimmed and lower-cased. Never match on name alone.
* The only tool that may write is `drip-add-subscriber` on Drip, once per confirmed email, and only after Step 3. Do not call `drip-update-subscriber`, `drip-add-subscriber-to-campaign`, `drip-start-subscriber-workflow` or any other Drip action.
* Never call any Rillet tool whose name starts with rillet-add, rillet-update, rillet-apply, rillet-unapply, rillet-record, rillet-set or rillet-setup. Rillet is read-only in this prompt.
* Never re-subscribe someone who is unsubscribed in Drip. Drip updates an existing subscriber on add, so Step 2 must run first.

**Validation:**

* If there are no gaps, say so and stop without writing.
* If the user does not confirm, stop and write nothing.