Swap the SMS provider

• Interface: SmsTransport (send(to, body, idempotencyKey): Promise<SmsResult>) in apps/notifications/src/channels/sms/sms-transport.ts.
• Today: TwilioTransport (twilio.ts) in production; EmailSinkSmsTransport (email-sink.ts) delivers texts to Mailpit as emails to <digits>@sms.test locally.
• Selected in: createTransport() in apps/notifications/src/channels/sms/sms.module.ts by SMS_PROVIDER (unset in production: nothing is texted).
• Env: SMS_PROVIDER (twilio | email), TWILIO_ACCOUNT_SID, TWILIO_AUTH_TOKEN, TWILIO_FROM, TWILIO_API_URL (test hook only).

Swap

1. A class implementing SmsTransport. Map the provider's errors onto SmsResult: permanent when retrying can't help, suppress: "unsubscribe" when the recipient opted out, suppress: "invalid" for numbers that can't receive texts. The dispatcher records suppressions (DeliveryPolicy) so those numbers are never texted again.
2. Its name in the SMS_PROVIDER enum and its credentials in apps/notifications/src/env.ts, with a boot check that they're all set, plus .env.example and docs/environment.md. A test hook for its API URL is refused in production like TWILIO_API_URL.
3. A case in createTransport().

Tests

apps/notifications/test/fake-twilio.ts is the pattern: a local server answering like the provider, including its error cases. Write one for the new provider and cover success, opt-out and invalid numbers in apps/notifications/test/notifications.integration.test.ts. apps/notifications/src/channels/sms/email-sink.test.ts covers the local sink.

Finish

1. The verify skill.
2. Ask the reviewer agent to review the change, and the security-reviewer agent: a new implementation brings its own credentials and sends data somewhere new. If you wrote a migration, the migration-reviewer agent too.
3. Update the seam's row in the README's "Scaling path" table if what's "Now" changed.