Update cleanup internals documentation

docs/internal/storage.md

@@ -26,6 +26,19 @@ Storage errors use typed categories such as not found, already exists, invalid p
 
 Backends may wrap implementation-specific errors, but callers should receive storage errors where practical. Traversal can stop cleanly with `ErrStopWalk`.
 
+## Traversal helpers
+
+Backends own their traversal mechanics. The local adapter owns filesystem walking, the SSH adapter owns SFTP directory walking, and the S3 adapter owns object listing and pagination.
+
+`internal/storage` owns the shared callback emission rules used by backends:
+
+- context cancellation is checked before callback emission;
+- `WalkOptions.Limit` bounds the number of emitted entries;
+- `ErrStopWalk` stops traversal without becoming a caller-visible error;
+- callback errors are wrapped as storage walk errors.
+
+`storage.HasAny(ctx, backend, prefix)` provides the shared destination-content check. It calls `Walk` with non-recursive, limit-one traversal and stops after the first emitted entry.
+
 ## Deletion
 
 `DeleteManagedBundle` may delete listed managed outputs plus `.distributor.json`.