fix(review): add OpenCode Zen free fallback

[seonghobae]

Summary

• add anonymous OpenCode Zen free reviewers for public repositories, preferring deepseek-v4-flash-free and retaining North as fallback
• keep the existing public-repository privacy gate and strict review-control validation
• preserve reviewer tool calls; do not force response_format, which made OpenCode describe a Read call as text instead of executing it
• align the candidate-order comment with the final DeepSeek-first order and both free models privacy exceptions

Closes #648.
Unblocks ContextualWisdomLab/saju-caldav#14.
Supersedes #649 and #653 with the same reviewed final diff as one clean commit.

Runtime evidence

• probe 1 — path .github/workflows/opencode-review-dispatch.yml, line 3364: challenged whether a free JSON-capable reviewer could still use tools; in an isolated OpenCode run, DeepSeek V4 Flash Free invoked Read on a sample file and then returned exact JSON
• probe 2 — path .github/workflows/opencode-review-dispatch.yml, line 3720: challenged whether anonymous Zen could receive private-repository content; the is_private == false gate wraps both free candidates, and the contract test parses that exact ordering
• anonymous Zen endpoint smoke: DeepSeek V4 Flash Free returned exact JSON
• North remains the fallback

Verification

• full python3 -m pytest -q — 674 passed for the final diff
• tests/test_opencode_agent_contract.py — 37 passed on this commit
• Bash syntax, focused Ruff, actionlint -shellcheck=, and git diff --check — passed

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: ab8cc054-7d17-4faa-a3d7-08643d84c2fe

📥 Commits

Reviewing files that changed from the base of the PR and between 5472a38 and 105a244.

📒 Files selected for processing (2)

• .github/workflows/opencode-review-dispatch.yml
• tests/test_opencode_agent_contract.py

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[seonghobae]

Superseded by the staged bootstrap: #655 prepares the contract in one changed file; after it merges, a workflow-only PR will activate DeepSeek and a final DeepSeek-reviewed test PR will pin the new state.