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/skill-routing-e2e.test.ts

@@ -46,7 +46,7 @@ if (evalsEnabled && !process.env.EVALS_ALL) {
 
 // Apply EVALS_TIER filter (same logic as e2e-helpers.ts)
 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);