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/touchfiles-data.ts

@@ -1118,10 +1118,11 @@ export const E2E_TOUCHFILES: Record<string, string[]> = {
 };
 
 /**
- * E2E test tiers — 'gate' blocks PRs, 'periodic' runs weekly/on-demand.
+ * E2E test tiers — 'gate' blocks PRs, 'periodic' runs weekly/on-demand,
+ * 'marathon' keeps full start-to-finish flows in a non-blocking lane only.
  * Must have exactly the same keys as E2E_TOUCHFILES.
  */
-export const E2E_TIERS: Record<string, 'gate' | 'periodic'> = {
+export const E2E_TIERS: Record<string, 'gate' | 'periodic' | 'marathon'> = {
   'ship-skipped-queued-finding': 'gate',
   'investigate-owned-completion': 'gate',
   'investigate-owned-abort': 'gate',