Resolve one customer by customer_id, lead_id, phone, email or external_id, or list customers with opaque cursor pagination (leads:read).

A customer is the durable contact record a business has talked to. customer_id and lead_id are the same value today and both are accepted, so a consumer holding either identifier resolves the same record. Supplying any ONE lookup key switches the endpoint to lookup mode and returns a single customer object (404 when nothing in the calling workspace matches — a value that exists in another workspace is reported identically, so this cannot be used to probe the platform). Supplying two lookup keys is a 400 rather than a silent precedence rule. Without a lookup key the endpoint lists customers ordered by the immutable created_at; page with the opaque next_cursor and synchronize with updated_since. A cursor is bound to the tenant scope, filters and sort order that produced it and is rejected against any other request.