feat: add reusable workflow definition graphs

docs/CONCEPT.md

@@ -95,7 +95,17 @@ Permit-to-payment MVP:
 
 ## MVP Slice
 
-The first implementation should provide:
+The first executable slice now provides:
+
+- a versioned, workflow-specific graphical node library
+- shared Core graph DTO and validation primitives also consumed by Dataflow
+- workflow constraints that permit loops while requiring one trigger and one or
+  more outcomes
+- trigger, activity, review, decision, wait, module-action, Dataflow, and outcome
+  nodes
+- API discovery and validation endpoints
+
+The next execution slices should provide:
 
 - static workflow definition registration from configuration packages
 - create/read/list workflow instances