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/workflow-agent-kit/src/context.ts

@@ -21,6 +21,14 @@ function fail(message: string): never {
   throw new Error(message);
 }
 
+function readEdgePrompt(): string {
+  const value = process.env.UWF_EDGE_PROMPT;
+  if (value === undefined || value === "") {
+    fail("UWF_EDGE_PROMPT environment variable is required");
+  }
+  return value;
+}
+
 function walkChain(store: Store, schemas: AgentStore["schemas"], headHash: CasRef): ChainState {
   const headNode = store.get(headHash);
   if (headNode === null) {
@@ -133,7 +141,7 @@ export async function buildContext(threadId: ThreadId, role: string): Promise<Ag
   }
 
   const steps = await buildHistory(store, chain.stepsNewestFirst);
-  const edgePrompt = process.env.UWF_EDGE_PROMPT ?? null;
+  const edgePrompt = readEdgePrompt();
 
   return {
     threadId,
@@ -180,7 +188,7 @@ export async function buildContextWithMeta(
   }
 
   const steps = await buildHistory(store, chain.stepsNewestFirst);
-  const edgePrompt = process.env.UWF_EDGE_PROMPT ?? null;
+  const edgePrompt = readEdgePrompt();
 
   return {
     threadId,

packages/workflow-agent-kit/src/types.ts

@@ -13,11 +13,10 @@ export type AgentContext = ModeratorContext & {
    */
   outputFormatInstruction: string;
   /**
-   * Edge prompt from the graph transition that led to this role.
-   * null on first entry (use full role definition), non-null on re-entry
-   * (use as continuation instruction from moderator).
+   * Edge prompt from the graph transition that led to this role (UWF_EDGE_PROMPT).
+   * Phase 2 will use visit history to choose full role definition vs continuation.
    */
-  edgePrompt: string | null;
+  edgePrompt: string;
 };
 
 export type AgentRunResult = {