feat: add Twilio SMS, voice, and verification emulator

[mvanhorn]

Summary

Add @emulators/twilio package with full Twilio REST API emulation for SMS, voice calls, and the Verify API (2FA verification codes). Includes an inspector UI with tabs for messages, verifications, and calls.

Why this matters

Nearly every app with user authentication needs SMS verification or 2FA. Testing these flows currently requires hitting production Twilio (costs money per SMS, slow, unreliable in CI) or mocking at the HTTP level. No existing API emulator covers Twilio.

With this emulator, the full auth flow can run locally: OAuth sign-in (GitHub/Google emulator) -> phone verification (Twilio emulator) -> welcome email (Resend emulator). All without network access.

Changes

New package: @emulators/twilio

Messages API (/2010-04-01/Accounts/:sid/Messages.json):

• POST to send SMS (generates realistic SIDs, tracks segments)
• GET to list messages
• GET /:sid to fetch a single message

Calls API (/2010-04-01/Accounts/:sid/Calls.json):

• POST to create a call record
• GET to list calls

Verify API (/v2/Services/:sid/Verifications and VerificationCheck):

• POST Verifications to generate a 6-digit code (configurable length)
• POST VerificationCheck to validate the code
• Codes expire after 10 minutes
• Wrong codes leave status as "pending" rather than consuming the attempt

Inspector UI (/):

• Tabbed interface (Messages, Verify, Calls) using shared renderInspectorPage
• Verification codes displayed in plain text for easy testing

Seed config:

twilio:
  account_sid: AC_test_account
  phone_numbers: ["+15551234567"]
  verify_services:
    - sid: VA_test_service
      friendly_name: "My App Verify"

Integration

• Registered twilio in SERVICE_REGISTRY (packages/emulate/src/registry.ts)
• Added @emulators/twilio as dev dependency of emulate
• Updated README.md with Twilio in the service list
• Created skills/twilio/SKILL.md with endpoint docs and examples

Demo

Shows sending an SMS, creating a verification code, and checking the code (approved). All local, no real SMS sent.

Testing

6 tests pass (4 existing + 2 new for Twilio):

• twilio sends SMS and lists messages - Messages API
• twilio verify sends code and checks it - Verify API with code extraction from inspector UI

This follows the same pattern as our previous service emulator PRs (#4 Stripe, #7 Resend, #10 Slack).

This contribution was developed with AI assistance (Claude Code).

[vercel]

@mvanhorn is attempting to deploy a commit to the Vercel Labs Team on Vercel.

A member of the Team first needs to authorize it.

[vercel]

README documents incorrect port numbers for AWS (4006 instead of 4007) and Twilio (4007 instead of 4012), and is missing 5 services (Okta, Resend, Stripe, MongoDB Atlas, Clerk) from the port list.