Verify Zendesk adapter against a live sandbox

## 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.

Start with src/voxforge/infrastructure/providers/support/zendesk.py and tests/unit/test_zendesk_ticketing_provider.py, then compare scripts/prove-real-voice.sh for the expected proof format. Use a Zendesk trial or sandbox to verify create, lookup, and duplicate-tag behavior, and record sanitized evidence, required environment variables, and a manual runbook in docs/product/ or known-limitations without committing credentials.

Written by the indexing model from the issue text.