Address PR review feedback

- Consolidate the PR changesets into one effect patch changeset
- Export Entity.KeepAliveHandler and use it from platform-cloudflare
- Return Effects from the entity mailbox operations
- Use services with optional access instead of References for the
  entity reply context
- Build mutateAndWake inside a single Effect.suspend

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

Author: tim-smart

.changeset/cloudflare-cluster-cron.md

@@ -0,0 +1,16 @@
+---
+"effect": patch
+---
+
+Add the `@effect/platform-cloudflare` package, running Effect Cluster on
+Cloudflare Workers and Durable Objects.
+
+One entity instance is one Durable Object with its SQLite storage as the
+system of record. The package provides the four Durable Object classes
+(entity, workflow, durable queue, singleton), the length-prefixed entity name
+encoding, and `CloudflareCluster.layer`, which wires the cluster `Sharding`
+service, the `WorkflowEngine`, and the `PersistedQueueFactory` from the
+same-Worker namespace bindings. The `Entity`, `Workflow`, `Activity`,
+`DurableClock`, `DurableQueue`, `Singleton`, and `ClusterCron` user APIs are
+unchanged on this path; every `DurableClock` is durable through the object's
+alarm.