Add remote storage backend

docs/runbooks/s3-archive-foundations.md

@@ -12,11 +12,14 @@ This runbook documents the currently implemented storage/archive foundations and
 - promotion rule validation for safe relative paths
 - run ID generation and path/key helper functions
 - manifest run/path identity fields
+- remote storage backend layer:
+  - object-store interface (`List`, `Download`, `Upload`, `Exists`)
+  - fake backend for deterministic tests
+  - S3-compatible backend using AWS SDK v2
+  - config-based backend construction helper
 
 ## Not Implemented Yet
 
-- real S3 backend integration
-- AWS SDK wiring
 - prepare-stage S3 object listing or download
 - archive-stage S3 upload or promotion writes
 - writing `current/manifest.json` or `current/run_id.txt` in S3
@@ -29,4 +32,4 @@ This runbook documents the currently implemented storage/archive foundations and
 
 ## Next Implementation Target
 
-Build the storage backend layer that can list/download/upload S3 objects through a fake-tested interface, then wire prepare/archive stages to use that backend.
+Use the storage backend layer in prepare-stage session audio discovery/download flow, while preserving local audio input support.