Add secrets directory credential resolver

docs/policy/architecture.md

@@ -166,7 +166,7 @@ For example, one destination may publish source files only as a long-term archiv
 
 ## Backend Abstraction
 
-Sources and destinations use the same storage abstraction. Current runtime execution uses the local filesystem backend. Additional storage backends should be peer implementations behind the same interface, and any backend-specific execution limitation must be documented.
+Sources and destinations use the same storage abstraction. Current runtime execution uses the local filesystem and SSH/SFTP backends. Additional storage backends should be peer implementations behind the same interface, and any backend-specific execution limitation must be documented.
 
 Application logic must interact with storage through internal backend interfaces. Backend-specific behavior belongs in adapter packages. Pipeline, bundle, state, publish, and transform packages must not import service-specific or filesystem adapter implementation details.
 
@@ -194,6 +194,7 @@ Use this current layout unless the project has a documented reason to differ:
 - `internal/state`: `.distributor.json` parsing, validation, comparison, and output metadata.
 - `internal/storage`: backend interfaces, shared path/resource types, backend registry, and storage errors.
 - `internal/adapters/local`: local filesystem backend.
+- `internal/adapters/ssh`: SSH/SFTP backend.
 - `internal/transform`: transform interfaces, registry, planning, and shared transform models.
 - `internal/transform/markdown`: Markdown-to-HTML implementation.
 - `internal/publish`: destination planning, reconciliation, safety checks, and publish execution.