test: add the non-blocking 'marathon' E2E tier

Full start-to-finish flows move out of the blocking lanes. E2E_TIERS and
E2ETier gain 'marathon'; describeE2ETier('marathon') is enabled only when
EVALS_TIER=marathon, so the gate/PR and periodic lanes (and the gate census)
never run those cases. The PR profile accepts marathon ids as scheduled
elsewhere and defers them with their own reason, even on full fallback.

test/helpers/e2e-helpers.ts

@@ -116,9 +116,10 @@ export let selectedTests: string[] | null = resolveModuleSelection(
 // EVALS_TIER: filter tests by tier after diff-based selection.
 // 'gate' = gate tests only (CI default — blocks merge)
 // 'periodic' = periodic tests only (weekly cron / manual)
+// 'marathon' = full end-to-end flows only (non-blocking marathon lane)
 // not set = run all selected tests (local dev default, backward compat)
 if (evalsEnabled && process.env.EVALS_TIER) {
-  const tier = process.env.EVALS_TIER as 'gate' | 'periodic';
+  const tier = process.env.EVALS_TIER as 'gate' | 'periodic' | 'marathon';
   const tierTests = Object.entries(E2E_TIERS)
     .filter(([, t]) => t === tier)
     .map(([name]) => name);