Run the Brain-gated e2e tier on every Engine PR, blocking

Context

Phase 2 of epic #19655, graduated from D#19599. This leaf is blocked by #19656, the dev-push burn-in. It starts only once that burn-in's receipts show hosted time per shard within the ceiling, together with a green streak on dev in which every red is dispositioned.

The Problem

The Brain-gated tier exists to catch dock and multi-window regressions before merge. After #19559, a regression sat on dev for about 14 hours. The tier helps only if it runs on PRs and blocks when it fails. A non-required check is not an alternative, because current consumers (validateMergeReady and the review audit) already treat any red as blocking.

The Architectural Reality

• The impact boundary: classify-test-scope.yml already holds the E2E impact boundary: src/, navigated apps and examples, themes and build scripts, Playwright support, harness apps, dependencies, and the workflow and classifier themselves. externalBrainSelection.mjs classifies fixture dependence, not transitive impact.
• Merge readiness: Brain's validateMergeReady reports nonRequiredFailures as blocking, and compareRequiredAndEmittedContexts reports absent required contexts.
• The job: the burn-in job from #19656 supplies provisioning, the Brain-tip resolution and the receipt fields.

The Fix

• Add pull_request to the job from #19656. The job always reports, and chooses full or smoke inside itself.
• Full tier when the E2E impact boundary matches, or when grid, AI-client or NL fixture/spec/config files change. Smoke subset otherwise.
• The summary records the selected population, so a green smoke is never read as full-tier evidence.
• A red blocks exactly as any red does. No continue-on-error.
• WhiteboxE2E.md states that the tier gates Engine PRs.

Contract Ledger

Acceptance Criteria

• The job runs on every pull_request to dev, always reports, and selects full or smoke inside itself.
• The full tier runs when the E2E impact boundary matches or grid, AI-client or NL fixture/spec/config files change; otherwise a smoke subset runs. The summary records the selected population.
• A deliberately failing run, carried through the real reporting path, stays red and is refused by validateMergeReady. No continue-on-error exists on the path.
• The PR links #19656's burn-in receipt on #19655: hosted time per shard within the ceiling, and a green streak on dev with every red dispositioned (#17844's set included).
• WhiteboxE2E.md states that the tier gates Engine PRs.

Out of Scope

Provisioning and receipts (#19656), option B (D#18775), and headed-only specs (local, at the cut).

Avoided Traps

• Masking reds with continue-on-error or an advisory contract the consumers haven't adopted.
• A path-filtered required check: it strands PRs, as neomjs/neo-agent-brain#972 did.

Related

#19655 (epic) · #19656 (blocked by) · D#19599 · #17844

unowned-rationale: blocked by #19656. Ada stewards the epic.

Origin Session ID: daac81df-5205-405e-a3af-aec56e5cca45 Retrieval Hint: "Brain-gated e2e tier every PR blocking full smoke selection classify-test-scope"