feat: per-call response verification at the proxy (v0.2.0)

[Mhemd139]

The implementation commit that PR #13 was merged without - the merge landed while the branch history was being cleaned up, so main received the spec docs only. This PR carries the single squashed implementation commit; the spec file is identical on both sides, so the diff is code + tests only.

What

Every tools/call response the proxy forwards is verified against a per-tool reference: declared outputSchema, else the core fingerprint (intersection of baselined probe fingerprints), else counted unverified. Output-side BREAKING classes (missing field, structural retype, forbidden null, pinned value mismatch) block the response and response-quarantine the tool; scalar retypes record DEGRADED and forward; extra fields pass.

• covenant/verify.py - pure verifier (Layer 0)
• two-bucket quarantine: sync() owns schema only; POST /covenant/refresh clears the response bucket; /covenant/status shows sources
• SSE interception: only the matching result frame is held (one frame''s latency); notifications stream through; 1 MiB cap; GET listen stream untouched
• verifier exceptions forward original bytes - inspection failure never drops traffic
• covenant proxy --observe for monitor-only rollout
• snapshot embeds expect pins into the lock; lock re-snapshotted (diff is exactly the pin)
• new metric covenant_response_verifications_total{tool, outcome}
• COVENANT_SHEKEL_DRIFT demo lever: currency swap the pin catches per-call

Tests

37 new tests; full suite, ruff, and strict mypy green.

🤖 Generated with Claude Code

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

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Run ID: 254c136f-082d-4b7a-b414-11f740c9e694

📥 Commits

Reviewing files that changed from the base of the PR and between caaae1e and 4bfd294.

📒 Files selected for processing (14)

• covenant.lock.json
• covenant/cli.py
• covenant/proxy/metrics.py
• covenant/proxy/quarantine.py
• covenant/proxy/server.py
• covenant/verify.py
• docs/specs/2026-07-26-covenant-per-call-response-verification-design.md
• examples/mcp_server.py
• tests/test_metrics.py
• tests/test_proxy.py
• tests/test_proxy_store.py
• tests/test_proxy_verify.py
• tests/test_quarantine.py
• tests/test_verify.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch spec/per-call-verification

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