feat(alerts): add SMS channel with Twilio integration - Issue #41 (59 tests)

Implements SMS alert delivery using Twilio API:
- SMSFormat enum (plain, compact, detailed)
- SMSStatus enum (queued, sending, sent, delivered, failed, etc.)
- SMSConfig for Twilio credentials and settings
- SMSMessageResult for delivery tracking
- SMSBatchResult for multi-recipient sends
- SMSMessageFormatter with priority indicators
- SMSChannel implementing AlertChannel protocol
- create_sms_channel factory function

Features:
- E.164 phone number validation
- Three formatting styles (plain, compact, detailed)
- Priority indicators ([!], [!!!]) for high/critical alerts
- Category prefixes (TRD, RSK, SYS, MKT, etc.)
- SMS segment counting (GSM-7 vs Unicode)
- Messaging service SID support
- Priority filtering (send only HIGH+ alerts)
- Batch sending to multiple recipients
- Retry logic with exponential backoff
- Server error retry vs client error fail-fast
- Latency tracking for performance monitoring
- Status callback URL support
- Connection test functionality

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>