Verify Zendesk adapter against a live sandbox #24

Summary

Zendesk adapter is implemented and unit-tested. It is not production-proof until someone hits a real sandbox.

Context

• PM tasklist Stage 19
• src/voxforge/infrastructure/providers/support/zendesk.py
• tests/unit/test_zendesk_ticketing_provider.py
• scripts/prove-real-voice.sh is the voice analogue; ticketing needs its own checklist

Scope

• Create/lookup/duplicate-tag behavior against a Zendesk trial/sandbox
• Record sanitized evidence (ticket id, no tokens) in docs/product/ or known-limitations
• Document required env vars and a manual runbook
• Do not commit subdomain credentials

Notes

Maintainer must provide sandbox credentials via GitHub environment secrets if this becomes CI. Default is a documented manual proof.