Restructure the SMS channel extension to follow the voice-call pattern:
- Rename from "plivo" to "sms" (technology-based naming)
- Add provider abstraction (SMSProvider interface)
- Implement PlivoProvider and MockProvider
- Support for future providers (Twilio, etc.)
This makes the extension more extensible and consistent with
the voice-call extension architecture.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
