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

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

@@ -624,12 +624,14 @@ function resolveAgentConfig(
   return agentConfig;
 }
 
-function spawnAgent(agent: AgentConfig, threadId: ThreadId, role: string, edgePrompt: string | null): CasRef {
+function spawnAgent(
+  agent: AgentConfig,
+  threadId: ThreadId,
+  role: string,
+  edgePrompt: string,
+): CasRef {
   const argv = [...agent.args, threadId, role];
-  const env = { ...process.env };
-  if (edgePrompt !== null) {
-    env.UWF_EDGE_PROMPT = edgePrompt;
-  }
+  const env = { ...process.env, UWF_EDGE_PROMPT: edgePrompt };
   let stdout: string;
   try {
     stdout = execFileSync(agent.command, argv, {

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

@@ -55,11 +55,16 @@ function isJsonSchema(value: unknown): value is JSONSchema {
 function normalizeGraph(graph: Record<string, Transition[]>): Record<string, Transition[]> {
   const result: Record<string, Transition[]> = {};
   for (const [node, transitions] of Object.entries(graph)) {
-    result[node] = transitions.map((t) => ({
-      role: t.role,
-      condition: t.condition ?? null,
-      prompt: t.prompt ?? null,
-    }));
+    result[node] = transitions.map((t) => {
+      if (typeof t.prompt !== "string" || t.prompt.trim() === "") {
+        fail(`graph[${node}] transition to "${t.role}": prompt is required (non-empty string)`);
+      }
+      return {
+        role: t.role,
+        condition: t.condition ?? null,
+        prompt: t.prompt,
+      };
+    });
   }
   return result;
 }

packages/cli-workflow/src/validate.ts

@@ -44,6 +44,8 @@ function isTransition(value: unknown): boolean {
   const condition = value.condition;
   return (
     typeof value.role === "string" &&
+    typeof value.prompt === "string" &&
+    value.prompt.trim() !== "" &&
     (condition === null || condition === undefined || typeof condition === "string")
   );
 }