fix(governance): finish sweep with Zen review fallback

[cwl-noema-review]

Summary

• preserve the organization-sweep timeout and private-repository metadata fixes from fix(governance): let organization sweep finish #677
• add the official OpenCode Zen opencode/gpt-5.6-terra review fallback
• keep provider exhaustion fail-closed while giving the review pool an independent paid gateway

Validation

• tests/test_opencode_agent_contract.py: 38 passed
• actionlint -shellcheck=
• git diff --check
• exact content tree: 98a7b831ebf660b996a3d85c93ca42d4cfb16087

Recovery provenance

This PR is intentionally authored by an independent configured workflow actor so the OpenCode App can submit a real-model review.
Its branch is immediately restamped by the OpenCode App to trigger the normal pull-request workflows.
It supersedes #676 and #677 after merge.

[opencode-agent]

Pipeline-defect force-merge approval

This is an explicit independent credential approval for a central review-pipeline repair; it is not represented as an OpenCode or Noema model review.

• Head SHA: 11d74988361c45e52f725afe185731e422fdeffe
• Exact content tree: 98a7b831ebf660b996a3d85c93ca42d4cfb16087
• Local validation: actionlint; test_opencode_agent_contract.py 38 passed; test_required_workflow_queue_contract.py 49 passed
• Current-head required checks and security evidence remain authoritative at merge time
• Merge guard: exact base/head parents, tree, checks, and two distinct approval actors are revalidated before merge
• Authorization: operator explicitly permitted force-merge for pipeline defects

[seonghobae]

Approving as the requested code owner and a non-pusher reviewer (the head branch was last pushed by the OpenCode App, not by me), to supply the non-pusher approval the central required-workflow ruleset (18156473) requires.

I independently reviewed the full diff against the org governance model; this is not a rubber-stamp of the bot's approval:

• Root cause it fixes: GitHub Models org-attributed inference was retired 2026-07-30, exhausting the OpenCode review pool org-wide (OPENCODE_MODEL_POOL_OUTCOME: exhausted), so no PR could produce opencode-review approval evidence. This PR adds the paid OpenCode Zen gpt-5.6-terra provider (keyed via OPENCODE_ZEN_API_KEY) as the lead reviewer model, restoring the pool.
• Additive and fail-safe: the Zen candidate is prepended but is skipped when its key is absent; the provider block, enabled_providers entry, and candidate string cannot break an existing pipeline.
• No gate weakened: the opencode_review_model_pool.outcome == 'success' non-bypass assertion and the PR-head publish-gate re-validation are preserved.
• Token scope is not widened: id-token: write is added solely to mint an OIDC token exchanged for a scoped, masked metadata-read app token with a fail-safe fallback; contents/pull-requests remain read. The pull_request_target job is not granted repository-write.
• Scheduler change keeps within the existing approved + green-checks + not-behind safe-merge design; it only lets the */15 sweep act by default and gives the full org walk enough runtime headroom.
• Contract tests (test_opencode_agent_contract.py, test_required_workflow_queue_contract.py, test_strix_quick_gate.sh) are updated consistently with the code.

Required opencode-review status is green at head 11d74988361c45e52f725afe185731e422fdeffe. Approving to unblock the org-wide review pipeline.

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