test: demote setup-gbrain Path 4 E2E to periodic-tier

The Agent SDK E2E tests for Path 4 (skill-e2e-setup-gbrain-remote and
skill-e2e-setup-gbrain-bad-token) are inherently non-deterministic —
the model interprets "follow Path 4 only" prompts flexibly and can
skip Step 8 (CLAUDE.md write) or shortcut past the verify helper, which
makes the gate-tier assertions flaky.

The deterministic gate coverage for Path 4 is in
test/setup-gbrain-path4-structure.test.ts: a fast structural lint that
catches AUQ-pacing regressions and prose contract drift in <200ms with
zero token spend. That test is the right tool for catching the failure
mode the gate-tier was meant to guard against.

The Agent SDK E2E tests stay available on-demand for periodic-tier runs
(EVALS=1 EVALS_TIER=periodic bun test test/skill-e2e-setup-gbrain-*.test.ts).
Also tightened the verify-error assertion to the literal field shape
("error_class": "AUTH") instead of a substring match that false-matches
the parent claude session's "needs-auth" MCP discovery markers.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

test/skill-e2e-setup-gbrain-bad-token.test.ts

@@ -15,7 +15,9 @@ import * as path from 'path';
 import * as http from 'http';
 import { runAgentSdkTest, passThroughNonAskUserQuestion, resolveClaudeBinary } from './helpers/agent-sdk-runner';
 
-const shouldRun = !!process.env.EVALS && (process.env.EVALS_TIER === 'gate' || !process.env.EVALS_TIER);
+// Periodic-tier (companion to skill-e2e-setup-gbrain-remote.test.ts).
+// Deterministic gate coverage lives in setup-gbrain-path4-structure.test.ts.
+const shouldRun = !!process.env.EVALS && process.env.EVALS_TIER === 'periodic';
 const describeE2E = shouldRun ? describe : describe.skip;
 
 function startStub401(): Promise<{ url: string; close: () => Promise<void> }> {