Add remote storage backend

architecture.md

@@ -32,11 +32,15 @@ Implemented:
   - S3 session/run/current key builders
   - campaign/session/run local work/spool path helpers
   - manifest run/path identity fields (`campaign`, `run_id`, local and S3 prefixes)
+- remote storage backend layer:
+  - narrow object-store interface (`List`, `Download`, `Upload`, `Exists`)
+  - fake storage backend for deterministic tests (no network dependency)
+  - S3-compatible backend using AWS SDK v2
+  - config-based object-store construction helper
 
 Still placeholder/future:
 
 - `archive` stage behavior
-- real S3 storage backend (list/download/upload/exists)
 - prepare-stage S3 audio download behavior
 - archive-stage S3 upload/promotion behavior
 - `notify` stage behavior
@@ -157,6 +161,15 @@ Cross-config validation scope:
 - `pipeline.storage.s3.bucket` is required only when an S3-dependent feature is explicitly configured (for current foundations, that includes `session.inputs.audio_s3`, and archive upload intent when using `storage.backend: s3`)
 - no AWS credentials are stored in Narratio config; credential resolution remains an external runtime concern
 
+Remote object-store backend scope:
+
+- remote storage APIs are isolated to `internal/adapters/storage`
+- AWS SDK types remain contained within the S3 backend implementation package
+- S3 key/session path semantics remain outside the backend, with this invariant:
+  - callers pass full bucket-relative object keys
+  - backend methods do not prepend `root_prefix` or infer campaign/session/run paths
+- the backend layer is available for future prepare/archive usage, but no stage currently invokes `List`/`Download`/`Upload`/`Exists`
+
 `pipeline.scriptorium` is optional. Existing pipelines without Scriptorium continue to work.
 
 `pipeline.trim` is optional. Existing pipelines without trim config continue to work.