fix(opencode): preserve audit chain verification across rollover eviction

[Peldan]

Description

The OpenCode adapter's audit log truncates to MAX_ENTRIES via .slice(-MAX_ENTRIES) on every append. That drops the GENESIS-anchored head, but verifyAuditEntries requires entries[0].previousHash === GENESIS_HASH, so once the log rolls over verification fails permanently. Because appends verify first and governance fails closed (denyOnPolicyError), every subsequent tool and prompt decision is then denied — the plugin bricks after MAX_ENTRIES entries.

Port the seam-hash re-anchoring already applied to the Go/Rust/TypeScript implementations in #1838 (which did not touch the OpenCode adapter): on eviction, retain the last evicted entry's hash as a seam and re-anchor the surviving chain to it in verifyAuditEntries. The seam is persisted in a backward-compatible way — logs that never roll over stay bare arrays; rolled-over logs use { seamHash, entries }; and an already-truncated legacy bare array is recovered by adopting its head's previousHash as the seam. Adds tests covering rollover, legacy recovery, format stability, and that content tampering is still detected.

Affected package: agent-governance-opencode (@microsoft/agent-governance-opencode).

Type of Change

• Bug fix (non-breaking change that fixes an issue)

Package(s) Affected

• agent-governance

Checklist

• I have added tests that prove my fix/feature works

AI Assistance

• I can explain every meaningful change in this PR: what it does, why, and what tradeoffs were considered
• I have run tests and verification appropriate for this change
• No part of this PR was autonomously submitted by an AI agent without my review
• I have not used AI to generate review comments on others' PRs

If AI tools materially shaped this change, briefly note what was used:
AI assistance was used to confirm the bug with a standalone reproduction (a post-truncation appendAuditEntry throws failed hash-chain verification), draft the fix, and clarify the PR description. All changes reviewed and owned by me.

IP, Patents, and Licensing

• This contribution does not implement patent-pending or patent-encumbered techniques
• This contribution does not require an NDA or licensing agreement to understand or use
• Any AI tools used have terms compatible with the MIT License

Related Issues

Related: #1838

[github-actions]

PR Review Summary

Check	Status	Details
🔍 Code Review	⚠️ Missing	No current-run comment
🛡️ Security Scan	⚠️ Missing	No current-run comment
🔄 Breaking Changes	⚠️ Missing	No current-run comment
📝 Docs Sync	⚠️ Missing	No current-run comment
🧪 Test Coverage	⚠️ Missing	No current-run comment

Verdict: ⚠️ AI review incomplete; ready for human review

AI review comments are untrusted advisory output. The summary reports workflow-generated completion status only, not model-authored pass/fail claims.

[github-actions]

Welcome to the Agent Governance Toolkit! Thanks for your first pull request.
Please ensure tests pass, code follows style (ruff check), and you have signed the CLA.
See our Contributing Guide.

[github-actions]

🤖 AI Agent: contributor-guide — View details

AI-generated review output. Treat it as untrusted analysis and verify before acting.

Welcome, and thank you for your contribution!

Your detailed explanation and thorough test coverage are impressive. Great job addressing backward compatibility and ensuring robust verification logic.

Before merging, please:

1. Confirm that all new tests pass in the CI pipeline.
2. Ensure the updated audit log format is documented for users in the relevant README or documentation.

For guidance, refer to CONTRIBUTING.md.

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