fix(security): verify downstream Semgrep artifacts

[djm81]

Superseded by #774 so the unchanged hosted Requirements gate can capture actual test-only RED before implementation on that branch. This attempt's signed commits, hosted failures and artifacts remain retained. No commits were rewritten and nothing was merged.

Why

Semgrep's upstream metadata pins MCP 1.29.0, which fails the security audit for CVE-2026-59951. This change builds a distinct downstream wheel with MCP 1.30.0 and preserves every upstream runtime byte.

What changed

• Add deterministic wheel verification and fail-closed admission against exact protected policy, artifact hashes and GitHub provenance.
• Bind complete audit coverage to the actual installed graph without dropping the downstream identity.
• Add 21 native platform/Python cells with paired upstream controls, full repository SAST and real MCP transport/lifecycle regressions.

Scope

• Covers candidate build, verification and admission policy infrastructure. Closes [Change] Build and admit a downstream Semgrep distribution with fixed MCP #772 when completed.
• Production admission starts empty. Protected producer bootstrap and production dependency adoption remain blocked and require separate reviewed evidence.
• Existing dependency pins, security audits, provider checks and release gates remain unchanged. No merge or release is requested.

Blast Radius

The new workflow runs only for this change's paths. Candidate jobs have read-only permissions and cannot grant production authority. The current frozen MCP audit remains a release blocker until protected adoption is possible.

Verification

• Exact signed authored snapshot has 40 AssertionError failures, zero errors/skips; implementation passes all 40 mapped regressions. The owner accepted the exact mapping.
• Local macOS arm64 Python 3.13 passes 11 MCP checks. Paired full SAST scans match over 306 files and pass the unchanged baseline and positive/negative controls.
• Normal signed hooks, strict OpenSpec, canonical types and FULL review pass. Narrow preinstallation/signature warnings are documented in REVIEW_EXCEPTIONS.md. Hosted 21-cell proof is pending and is not inferred from local probes.

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[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-16T21:30:09.264384Z	0ba968a	PR opened
🔒 Security Review	✅ Completed 2026-10-16T21:32:32.367901Z	0ba968a	PR opened

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📥 Commits

Reviewing files that changed from the base of the PR and between 729a53b and 0ba968a.

⛔ Files ignored due to path filters: 1

• openspec/changes/downstream-semgrep-admission/decision-trail.tsv is excluded by !**/*.tsv

📒 Files selected for processing: 16

• .github/workflows/downstream-semgrep.yml
• ci/downstream-semgrep-admissions.json
• ci/downstream-semgrep-recipe.json
• openspec/CHANGE_ORDER.md
• openspec/changes/downstream-semgrep-admission/REVIEW_EXCEPTIONS.md
• openspec/changes/downstream-semgrep-admission/TDD_EVIDENCE.md
• openspec/changes/downstream-semgrep-admission/design.md
• openspec/changes/downstream-semgrep-admission/proposal.md
• openspec/changes/downstream-semgrep-admission/requirements-evidence.yaml
• openspec/changes/downstream-semgrep-admission/requirements-proof/review-evidence.json
• openspec/changes/downstream-semgrep-admission/specs/downstream-semgrep-admission/spec.md
• openspec/changes/downstream-semgrep-admission/tasks.md
• scripts/downstream_semgrep.py
• scripts/verify_downstream_semgrep.py
• tests/unit/scripts/test_downstream_semgrep.py
• tools/downstream_semgrep/test_mcp_regressions.py

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[github-actions]

SpecFact CLI Validation Report

✅ All validations passed!
Duration: 25.40s
Checks: 4 total (3 passed) (1 skipped)