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/cli-workflow/src/commands/workflow.ts

@@ -58,6 +58,7 @@ function normalizeGraph(graph: Record<string, Transition[]>): Record<string, Tra
     result[node] = transitions.map((t) => ({
       role: t.role,
       condition: t.condition ?? null,
+      prompt: t.prompt ?? null,
     }));
   }
   return result;