farm: model discovery surfaces expired 'free promotion' models (401) — add entitlement pre-check #93

Found during the docs-site-mvp /ca:sprint --farm run (2026-06-18).

OpenCode Zen's /models catalog lists models the API key is not entitled to. minimax-m3-free and qwen3.6-plus-free are listed but /chat/completions returns HTTP 401 with "Free promotion has ended for ...". The canary cannot distinguish this from a model-quality failure and burns full attempts/timeouts on dead candidates (one earlier 7-model canary ran for minutes before being killed).

Workaround used: a fast direct screen (one tiny /chat/completions per candidate, 35s timeout) to drop 401 models before the real canary; big-pickle survived and passed.

Perma-fix candidates:

• Before the canary, do a cheap 1-token /chat/completions entitlement probe per candidate; drop any that return 401 with a clear "promotion ended / not entitled" note.
• Surface 401 distinctly from capability failure in canary-report.json.
• Cap per-candidate canary wall-clock so one slow/dead model can't dominate the probe.