Add remote storage backend

docs/roadmap/narratio-s3-archive-implementation-plan.md

@@ -95,11 +95,14 @@ Implemented in repository:
   - campaign/session/run local work and spool path helpers
   - manifest run/path identity fields
 - examples and tests for the above foundations
+- remote storage backend layer:
+  - object-store abstraction with `List`, `Download`, `Upload`, and `Exists`
+  - fake storage backend for deterministic, no-network testing
+  - S3-compatible backend built from `storage.s3` config
+  - backend construction helper from resolved config
 
 Not implemented yet:
 
-- real S3 backend
-- AWS SDK integration
 - prepare-stage S3 list/download behavior
 - archive-stage S3 upload behavior
 - promotion uploads
@@ -849,9 +852,9 @@ Expected commit:
 Add archive storage path configuration
 ```
 
-### Storage Backend Interface and S3 Backend
+### Storage Backend Interface and S3 Backend (Implemented)
 
-Implement:
+Implemented:
 
 - storage backend interface
 - object metadata type
@@ -859,7 +862,7 @@ Implement:
 - real S3 backend using AWS SDK or existing project dependency policy
 - backend construction from config
 
-No stage behavior yet.
+No prepare/archive stage behavior yet.
 
 Expected commit: