Operationalize or remove the paid OpenCode Zen fallback

[seonghobae]

Buyer-visible reliability gap

The central reviewer advertises opencode/gpt-5.6-terra as a paid fallback, but the latest exact-head evidence recorded on #698 shows OPENCODE_ZEN_API_KEY is not configured. Reordering an unavailable provider ahead of the free pool does not make review latency predictable and can misrepresent the operational fallback posture.

Completion criteria

Choose one explicit production state:

1. Operational paid fallback

   • provision OPENCODE_ZEN_API_KEY through the authorized repository/organization secret path;
   • add a credential-presence preflight that does not expose the value;
   • verify a bounded current-head review reaches Terra and returns a valid control artifact;
   • document cost ownership, quota, timeout, and failure behavior.

2. No paid fallback

   • remove Terra from the active candidate order and documentation;
   • keep the bounded public/free fallback order explicit;
   • ensure provider exhaustion remains fail-closed and diagnosable.

Do not merge an ordering-only change that implies the paid fallback is available before one of these states is proven.