feat: add messaging R2 polling producer

src/sample-portfolio/CLAUDE.md

@@ -22,6 +22,9 @@ Package root: `dev.caskeleton.sample.portfolio`.
   `RepoStatsAclMapper`), `adapter/identifier` (`UuidWorkLogIdFactory`).
 - Contract-test fixtures the app-bootstrap verification suite analyses
   (`DomainExceptionHandler`, `PortfolioErrorCode`, wire/contract tests).
+- Disposable messaging contract fixture: the exact `WorkLogReservedPayload` record, its static
+  application-core contract contribution, sample-owned schema/digest and golden vectors. It is not
+  runtime-discovered, and validator compatibility remains unproven until messaging Task 6.
 - Sample application collaborators consume invocation context through application-core ports and
   must not import SLF4J/MDC.
 
@@ -39,6 +42,8 @@ Package root: `dev.caskeleton.sample.portfolio`.
 - Production-required behaviour living here: deleting this module must not break the
   production build or runtime (the base `GlobalExceptionHandler`, envelope, and
   error-code contract live in `adapter:inbound:web` / `shared-contract`).
+- Treating a sample schema/contribution as an automatically discovered production registry entry,
+  or placing JSON mapper, physical topic, Kafka or bootstrap-server concerns in the contribution.
 
 ## Test