Add SSH SFTP backend support

docs/internal/app.md

@@ -27,7 +27,7 @@ Destination failures are collected while later destinations continue to run. Sou
 
 ## Backend and transform wiring
 
-The app-level backend factory registers only the local backend for execution. Config validation accepts other backend shapes, but `Run` can execute only local sources and local destinations.
+The app-level backend factory registers local and SSH backends for execution. Config validation accepts S3 shape, but `Run` cannot execute S3 sources or destinations.
 
 The app-level transform registry registers Markdown-to-HTML using `internal/transform/markdown`. Lower-level publish code receives a resolver and does not import concrete transform implementations.
 
@@ -45,7 +45,7 @@ Stdout write errors are returned immediately because the caller's requested outp
 
 `internal/app` coordinates packages but does not own manifest validation rules, destination state comparison, storage path rules, output planning, transform rendering, or backend-specific filesystem behavior.
 
-`Validate` and `Inspect` are local path commands. Remote execution wiring is outside current behavior.
+`Validate` and `Inspect` are local path commands. Remote execution wiring currently belongs to `Run`.
 
 ## Tests
 

docs/internal/config.md

@@ -19,6 +19,8 @@ Known-field checking rejects misspelled or unknown YAML keys before defaults and
 Defaults are applied in `ApplyDefaults`:
 
 - pipeline validation defaults `on_digest_mismatch` to `fail`;
+- SSH backend `port` defaults to `22`;
+- SSH backend `host_key_policy` defaults to `accept-new`;
 - destination publish policy defaults to source output only;
 - `transfer.on_destination_same` defaults to `skip`;
 - `transfer.on_destination_older` defaults to `replace`;
@@ -33,7 +35,9 @@ Validation requires at least one pipeline, slug-like unique pipeline ids, one so
 
 ## Executable support boundary
 
-Config validation accepts `local`, `ssh`, and `s3` backend shapes so config files can be validated as schemas. Runtime execution currently opens only local backends through `internal/app`.
+Config validation accepts `local`, `ssh`, and `s3` backend shapes so config files can be validated as schemas. Runtime execution opens local and SSH backends through `internal/app`; S3 remains accepted by validation but unavailable at execution.
+
+SSH config uses structured fields: `host`, optional `user`, optional `port`, `path`, optional `ssh_key_file`, optional `known_hosts`, and optional `host_key_policy`. `host_key_policy` accepts YAML booleans and strings and normalizes `true`/`strict`, `accept-new`, and `false`/`off`.
 
 The user-facing configuration reference is `docs/config.md`; this file documents package behavior for maintainers.
 

docs/internal/storage.md

@@ -14,7 +14,7 @@ Entries report a logical path, type, and size when available. Entry types are `f
 
 Core packages should depend on `internal/storage`, not adapter packages. Adapter-specific path handling stays behind backend implementations.
 
-The local adapter lives in `internal/adapters/local`. Runtime backend construction is wired through the app-level backend factory and storage registry. The fake backend lives in `internal/storage/fake` for tests and is not registered for runtime use.
+The local adapter lives in `internal/adapters/local`. The SSH/SFTP adapter lives in `internal/adapters/ssh`. Runtime backend construction is wired through the app-level backend factory and storage registry. The fake backend lives in `internal/storage/fake` for tests and is not registered for runtime use.
 
 ## Paths
 
@@ -30,10 +30,12 @@ Backends may wrap implementation-specific errors, but callers should receive sto
 
 Backends expose guarded managed deletion only. `DeleteManagedBundle` may delete listed managed outputs plus `.distributor.json`; it does not provide broad recursive deletion.
 
-## Local and fake backends
+## Local, SSH, and fake backends
 
 The local adapter maps logical paths to a configured filesystem root and keeps adapter-specific path handling behind the storage interface.
 
+The SSH adapter maps logical paths to a configured remote SFTP root. It uses native SSH and SFTP libraries, supports SSH agent and key-file authentication, applies host-key policies, rejects unsafe logical paths, reports symlink entries from `Lstat`, and limits deletion to managed targets.
+
 The fake backend is an in-memory implementation for package tests. It is not registered for runtime use.
 
 ## Tests
@@ -43,6 +45,7 @@ Before changing storage behavior, inspect tests under:
 - `internal/storage`
 - `internal/storage/fake`
 - `internal/adapters/local`
+- `internal/adapters/ssh`
 
 ## Invariants