fix(ecs): remove dead ECS_PAYLOAD_OBJECT_KEY_PREFIX export (review B1)

The `export const ECS_PAYLOAD_OBJECT_KEY_PREFIX = ''` constant was referenced
nowhere — ecsPayloadKey() (ecs-strategy.ts) builds `<task_id>/payload.json`
independently and already documents that layout in its own docstring. The dead
export was a "prefix" whose value was '' with a docstring describing a key
layout it didn't produce, and it raised the knip dead-code count, tripping the
ratchet inside `mise run build`. Delete it; ecsPayloadKey remains the single
source of truth for the key layout.

Author: isadeks

cdk/src/constructs/ecs-payload-bucket.ts

@@ -32,13 +32,6 @@ import { Construct } from 'constructs';
  */
 export const ECS_PAYLOAD_TTL_DAYS = 1;
 
-/**
- * Object-key prefix for ECS task payloads. Key layout:
- * ``<task_id>/payload.json``. Each task writes a single object under its own
- * task-id prefix; the orchestrator deletes it on terminal.
- */
-export const ECS_PAYLOAD_OBJECT_KEY_PREFIX = '';
-
 /**
  * Properties for the EcsPayloadBucket construct.
  */