Implement telephony ingress and PBX handoff aligned to conversational mesh standard

[mdheller]

Implement the telephony ingress lane in prophet-platform based on the merged conversational mesh standard set.

Scope:

• inbound telephony/PBX ingress surface
• canonical CallEventEnvelope emission
• call-state transitions and routing outcomes
• transcript and recording references where policy permits
• fallback paths for human handoff, callback queue, and voicemail

Acceptance criteria:

• a telephony ingress service or deployment lane exists in prophet-platform
• inbound call events can emit canonical CallEventEnvelope records
• call-state transitions are preserved as first-class events
• deterministic fallback paths are represented in runtime design
• validate and smoke checks remain green