Phone/SMS verification provider integration

[jeremymanning]

Parent: #7

Integrate a phone verification service (Twilio, Vonage, or equivalent) for SMS/voice code verification. Currently returns ProviderUnavailable.

Files: src/identity/phone.rs lines 17, 24
Requires: SMS provider account

[jeremymanning]

Resolved by PR #58 (spec 003-stub-replacement). All 77 tasks complete, 489+ tests passing, CI green on Linux/macOS/Windows.