fix: accept omitted condition in fallback transitions

Fallback transitions (last entry in graph node) omit the condition
field in YAML, resulting in undefined instead of null. The validator
and materializer now handle this:

- validate.ts: accept undefined as valid condition value
- workflow.ts: normalizeGraph() coerces undefined → null before CAS put

This was broken by the graph fallback pattern introduced in #370.

packages/cli-workflow/src/validate.ts

@@ -42,7 +42,10 @@ function isTransition(value: unknown): boolean {
     return false;
   }
   const condition = value.condition;
-  return typeof value.role === "string" && (condition === null || typeof condition === "string");
+  return (
+    typeof value.role === "string" &&
+    (condition === null || condition === undefined || typeof condition === "string")
+  );
 }
 
 function isStringRecord(value: unknown, itemCheck: (item: unknown) => boolean): boolean {