Add a doc-parity unit test pinning documented coverage thresholds to the enforced gates #1193

Problem: docs drift is being fixed per-instance, never as a class

10 of the last 100 closed issues were documentation-drift reports, and the same spots drift repeatedly:

• [guide] CONTRIBUTING.md says e2e coverage 'has no percentage thresholds' — CI gates it at --check-source 100 --check-source-regions 80 #1037: CONTRIBUTING.md said e2e coverage had 'no percentage thresholds' while CI gated at --check-source 100 --check-source-regions 80
• [guide] CONTRIBUTING.md and AGENTS.md still document the unit region gate as 94% after #1152 ratcheted it to 95% #1169 (now Docs drift: sync LAUNCH.md verification dates and coverage-gate numbers with current state #1189): CONTRIBUTING.md and AGENTS.md still say the unit region gate is 94% after test: ratchet the unit region gate to the measured 95 #1152 ratcheted it to 95%
• [guide] CONTRIBUTING.md pre-PR checklist and CI-failure guidance omit the new required check:audit gate #1160: CONTRIBUTING.md omitted the new check:audit gate
• [guide] SECURITY.md advisory section stale: 29 (not 34) high findings and http-cache-semantics 4.3.0 fix now published #1045, [sec-check] http-cache-semantics 4.3.0 patches GHSA-ch52-4w7c-c8xp, but the lockfile is still on 4.2.0 and SECURITY.md says no fix exists #1049: SECURITY.md advisory counts went stale twice

The repository already proves the fix pattern works where it is applied:

• tests/audit-gate.test.mjs:174 pins the audit allowlist to SECURITY.md — that drift class has not recurred since
• tests/workflow-scripts.test.mjs:355 pins --check-source-regions 91 to ci.yml
• tests/dev-environment.test.mjs pins the Node major across devcontainer/workflows/docs (test(dev-environment): pin package.json engines.node into the Node-major parity chain #1183)

But nothing pins the threshold numbers stated in CONTRIBUTING.md and AGENTS.md to the values the gate actually enforces (tests/coverage-gate-thresholds.test.mjs / ci.yml). As of today, AGENTS.md:40 and CONTRIBUTING.md:176 still say '94% regions' while the enforced gate is 95 — the exact drift #1189 is about to fix for the second time.

Proposed fix

Add a unit test (e.g. tests/docs-threshold-parity.test.mjs) that:

1. Extracts the single source of truth for each gate (the --check-* flags asserted in tests/coverage-gate-thresholds.test.mjs and .github/workflows/ci.yml).
2. Asserts every place CONTRIBUTING.md, AGENTS.md, and .github/copilot-instructions.md states those numbers matches.

Then the next ratchet PR fails locally until the docs are updated in the same commit, and the recurring guide-issue class ends.

Completion criteria

• A unit test fails when any documented coverage threshold diverges from the enforced value.
• The test covers CONTRIBUTING.md, AGENTS.md, and copilot-instructions.md at minimum.
• Lands with or after Docs drift: sync LAUNCH.md verification dates and coverage-gate numbers with current state #1189 so the current 94-vs-95 drift does not fail it on arrival.