feat(protocol): add edge prompt to Transition + EvaluateResult (#402)

- Transition type gains prompt: string | null
- evaluate() returns EvaluateResult { role, prompt } instead of string
- normalizeGraph coerces prompt: undefined → null
- spawnAgent passes edge prompt via UWF_EDGE_PROMPT env
- AgentContext gains edgePrompt field

Refs #402

packages/workflow-protocol/src/types.ts

@@ -28,6 +28,7 @@ export type RoleDefinition = {
 export type Transition = {
   role: string;
   condition: string | null;
+  prompt: string | null;
 };
 
 export type ConditionDefinition = {