feat: make edge prompt required (Phase 1)

- Transition.prompt: string | null → string
- EvaluateResult.prompt: string | null → string
- AgentContext.edgePrompt: string | null → string
- CLI YAML validation rejects missing prompt
- All tests updated

Phase 2 will replace edgePrompt === null checks with findLastRoleIndex.

Refs #405, #406, #404

examples/analyze-topic.yaml

@@ -36,6 +36,8 @@ graph:
   $START:
     - role: "analyst"
       condition: null
+      prompt: "Analyze the topic in the task and produce a structured summary with key points."
   analyst:
     - role: "$END"
       condition: null
+      prompt: "Analysis complete. Finish the workflow."