feat: builtin agent session resume via deterministic message reconstruction (#426)

- StepRecord adds edgePrompt field (backward compat: defaults to "")
- StepNode CAS schema includes edgePrompt
- writeStepNode persists ctx.edgePrompt
- buildHistory exposes edgePrompt in StepContext
- buildBuiltinMessages reconstructs multi-turn moderator↔agent conversation:
  system = role prompt + output format (stable prefix)
  per prior visit: user (edgePrompt + inter-step summary) + assistant (output)
  current: user (edgePrompt + recent summary)
- Zero extra persistence — pure function of CAS chain
- Stable prefix for LLM prompt cache hits
- 10 builtin tests pass, all other package tests pass

packages/workflow-agent-kit/__tests__/build-continuation-prompt.test.ts

@@ -7,6 +7,7 @@ const reviewerStep: StepContext = {
   output: { approved: false, comments: "Missing tests" },
   detail: "2MXBG6PN4A8JR",
   agent: "uwf-hermes",
+  edgePrompt: "Review the developer's work.",
 };
 
 const developerStep: StepContext = {
@@ -14,6 +15,7 @@ const developerStep: StepContext = {
   output: { filesChanged: ["src/app.ts"], summary: "Initial fix" },
   detail: "1VPBG9SM5E7WK",
   agent: "uwf-hermes",
+  edgePrompt: "Implement the fix.",
 };
 
 describe("buildContinuationPrompt", () => {
@@ -26,6 +28,7 @@ describe("buildContinuationPrompt", () => {
         output: { plan: "revise approach" },
         detail: "7BQST3VW9F2MA",
         agent: "uwf-hermes",
+        edgePrompt: "Revise the plan.",
       },
     ];
 

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

@@ -102,6 +102,7 @@ async function buildHistory(
       output: expandOutput(store, step.output),
       detail: step.detail,
       agent: step.agent,
+      edgePrompt: step.edgePrompt ?? "",
     });
   }
   return history;

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

@@ -50,6 +50,7 @@ async function writeStepNode(options: {
   outputHash: CasRef;
   detailHash: CasRef;
   agentName: string;
+  edgePrompt: string;
 }): Promise<CasRef> {
   const payload: StepNodePayload = {
     start: options.startHash,
@@ -58,6 +59,7 @@ async function writeStepNode(options: {
     output: options.outputHash,
     detail: options.detailHash,
     agent: options.agentName,
+    edgePrompt: options.edgePrompt,
   };
   const hash = await options.store.put(options.schemas.stepNode, payload);
   const node = options.store.get(hash);
@@ -95,6 +97,7 @@ async function persistStep(options: {
     outputHash: options.outputHash,
     detailHash: options.detailHash,
     agentName: options.agentName,
+    edgePrompt: options.ctx.edgePrompt,
   });
 }