Adds TelephonyRegistrationService that:
1. On startup: queries platform for agent list, registers with the
telephony dispatcher at TELEPHONY_DISPATCHER_URL
2. Every 30s: sends heartbeat to keep registration alive (90s TTL)
3. On shutdown: deregisters (best-effort, TTL cleans up anyway)
4. On heartbeat failure: auto re-registers
Env vars:
TELEPHONY_DISPATCHER_URL — where to register (outbound to dispatcher)
TELEPHONY_CALLBACK_URL — where events come back (inbound to sidecar)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
