wire ts in the builder

docs/authoring_sketch.md

@@ -256,6 +256,9 @@ Today there are two different authoring paths:
   `Workflow`: child input schema, output schema, and workflow outcomes are
   copied into the boundary. Runtime execution still raises until native
   subgraph scopes are implemented.
+- `WorkflowBuilder.subgraph(...)` is the builder-facing version of
+  `subgraph_ref`: it appends the native boundary step and returns it as a
+  `StepRef` for `connect()` / `set_entry_point()`.
 
 In the future, a compiled workflow or subgraph can also be exposed as a reusable
 `NodeSpec`, likely by treating workflow input and output schemas as the node's

docs/superpowers/specs/2026-05-24-native-subgraphs-design.md

@@ -306,7 +306,7 @@ composition.
 Current helper:
 
 ```python
-child = subgraph_ref(
+child = parent.subgraph(
     id="run_child",
     workflow=child_builder.compile(),
     input=[input_from(state_path("request"), "request")],
@@ -314,10 +314,12 @@ child = subgraph_ref(
 )
 ```
 
-This copies the compiled child workflow contract into a core `SubgraphNode` but
-does not make the child executable yet. `workflow` is still a string reference
-inside the core model; higher layers need a structural workflow reference before
-saved/deployed workflow dependencies become stable.
+This copies the compiled child workflow contract into a core `SubgraphNode`,
+appends it to the builder, and returns the step for normal routing. It does not
+make the child executable yet. `workflow` is still a string reference inside the
+core model; higher layers need a structural workflow reference before
+saved/deployed workflow dependencies become stable. The lower-level
+`subgraph_ref(...)` helper exists for code that wants only the core step object.
 
 Possible API: