feat(email): add EmailClient for SDK email send preview

[jcpsimmons]

What this is

Preview-grade addition of a small EmailClient surface to customerio-node. One constructor, one method (send), one input type. Calls POST /v1/send/email. Pairs with the gated path in the Edge PR below.

Files

• lib/email.ts — EmailClient class. Exports SDK_PREVIEW_USER_AGENT_MARKER so the marker stays in sync with the server.
• lib/email-input.ts — SendEmailInput type + runtime validator.
• lib/request.ts — CIORequest gains an optional third constructor arg, userAgentSuffix. When omitted (the default for TrackClient/APIClient), the User-Agent header is byte-identical to before. When provided (by EmailClient), it's appended.
• test/email.ts — ava tests, 100% coverage on the new surface.
• test/request.ts — two new test cases for the userAgentSuffix path.
• examples/email-quickstart.ts — copy-paste demo.
• package.json — version 4.4.0 → 4.5.0 (additive bump).
• lib/version.ts — same version bump (kept in sync per existing test invariant).
• README.md — new "Sending email (preview)" section.

How it's used

import { EmailClient } from 'customerio-node';

const cio = new EmailClient({ apiKey: process.env.CIO_API_KEY! });

const { delivery_id } = await cio.send({
  to: 'user@example.com',
  from: 'verified@yourdomain.com',
  subject: 'Hello',
  body: '<p>Hi there</p>',
});

Server-side gate

The SDK sets User-Agent: Customer.io Node Client/4.5.0 CioEmailSDK-Preview. The matching Edge gate (sister PR) treats this as the "I'm using the preview" signal and 403s unless the workspace has the sdk_email_send_preview flag enabled. Any existing key on an enrolled workspace will work — no new key minting required.

What this is NOT

• Not a replacement for APIClient.sendEmail. That keeps working with the same User-Agent it always had.
• Not a TrackClient change.
• Not GA.

Sister PR

customerio/edge: feat(transactional): add gated User-Agent path for SDK email preview.

Tests

ava: 146 tests passed
coverage: 100% statements / 100% branches / 100% functions / 100% lines