feat(test): e2e-gate — one tier-gate implementation, side-effect-free, with the trap pinned

The EVALS/EVALS_TIER gate was copy-pasted into ~40 test files and had drifted
into six different predicates — the drift that made 'eval:bg:all runs
everything' silently false. test/helpers/e2e-gate.ts owns the semantics now:
describeE2ETier(tier) + e2eTierEnabled(tier), env read at call time, zero
side effects (the existing e2e-helpers module runs a ~30s claude ping at
import under EVALS=1, so the gate lives in its own module; purity is pinned
by tests that scan imports and comment-stripped source).

The unit matrix pins all four env combos — including EVALS=1 with EVALS_TIER
unset -> SKIP, the exact trap that made eval:bg:all a non-run. The
tier-alignment tripwire gains a second regex for the helper shape (old shape
still detected — stragglers can't hide), and the sharded paid runner's
PRE-SPAWN tier classifier learns the helper shape too: without that, every
gate-sharded run would have spawned all 28 periodic shards just to skip them,
each paying the e2e-helpers import ping (~15 min of dead wall clock in the
CI-blocking lane). Verified: gate runs exclude the 29 periodic files,
periodic excludes the 8 gate files — identical to pre-migration.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/helpers/e2e-gate.ts

@@ -0,0 +1,51 @@
+/**
+ * Whole-file E2E tier gate — the single definition of the
+ * `EVALS=1 && EVALS_TIER === '<tier>'` predicate that tier-gated paid test
+ * files used to copy-paste (~36 local copies before consolidation).
+ *
+ * This module MUST stay side-effect-free. It is imported at module scope by
+ * every tier-gated test file, including files the sharded paid runner
+ * (scripts/test-paid-shards.ts) spawns one-process-each — unlike
+ * test/helpers/e2e-helpers.ts, whose EVALS=1 module-scope work includes a
+ * ~30s `claude -p` connectivity ping, diff-based selection, and ~/.gstack
+ * pre-seeding. The only import allowed here is `bun:test`.
+ * test/helpers/e2e-gate.unit.test.ts enforces this with a source scan.
+ *
+ * Env is read at CALL time (the importing test file's module top-level), not
+ * captured at this module's load time, so the gate behaves identically under
+ * single-process `bun test` globs and the sharded runner's per-shard env.
+ *
+ * Static-grep consumers that must recognize the call shape
+ * `describeE2ETier('<tier>')` / `e2eTierEnabled('<tier>')` alongside the raw
+ * `EVALS_TIER === '<tier>'` predicate:
+ *   - test/e2e-tier-alignment.test.ts (HELPER_GATE_RE) — tier-alignment invariant
+ *   - scripts/test-paid-shards.ts classifyPaidTestFile — pre-spawn tier exclusion
+ */
+
+import { describe } from 'bun:test';
+
+export type E2ETier = 'gate' | 'periodic';
+
+/**
+ * True when this process should run whole-file-gated paid tests of `tier`:
+ * EVALS=1 AND EVALS_TIER exactly equals the tier.
+ *
+ * Deliberate consequence: EVALS=1 with EVALS_TIER unset is false for BOTH
+ * tiers. Tierless runs (`test:evals` / `eval:bg` / `eval:bg:all`) skip every
+ * tier-gated file and rely on diff-based per-test selection instead — that is
+ * the long-standing behavior of the copy-pasted predicates, pinned by
+ * test/helpers/e2e-gate.unit.test.ts.
+ */
+export function e2eTierEnabled(tier: E2ETier): boolean {
+  return !!process.env.EVALS && process.env.EVALS_TIER === tier;
+}
+
+/**
+ * `describe` when `e2eTierEnabled(tier)`, else `describe.skip`.
+ *
+ * Usage (module top-level of a tier-gated test file):
+ *   const describeE2E = describeE2ETier('periodic');
+ */
+export function describeE2ETier(tier: E2ETier): typeof describe | typeof describe.skip {
+  return e2eTierEnabled(tier) ? describe : describe.skip;
+}