fix(builtin-agent): don't delete session jsonl until process exits

Previously runBuiltinWithMessages deleted the session jsonl after each
run/continue call. This meant the createAgent retry mechanism (which
calls continue on frontmatter validation failure) would lose all
previous turn data — each continue started with an empty jsonl.

Now the session jsonl accumulates across run + continue calls, so the
final storeBuiltinDetail captures all turns. The jsonl file is left
behind for debugging; it's small and can be cleaned up on next startup.

Also add a workflow hint to the system prompt reminding the LLM to use
tools before outputting frontmatter, preventing premature text-only
responses on the first turn.

packages/workflow-agent-builtin/src/agent.ts

@@ -13,7 +13,7 @@ import { storeBuiltinDetail } from "./detail.js";
 import type { ChatMessage } from "./llm/index.js";
 import { BUILTIN_CONTINUE_MAX_TURNS, BUILTIN_MAX_TURNS, runBuiltinLoop } from "./loop.js";
 import { buildBuiltinMessages } from "./prompt.js";
-import { initSessionDir, removeSession } from "./session.js";
+import { initSessionDir } from "./session.js";
 
 const log = createLogger({ sink: { kind: "stderr" } });
 
@@ -62,7 +62,6 @@ async function runBuiltinWithMessages(
 
   if (loopResult.turnCount === 0) {
     log("5RWTK9NB", "no turns produced, returning empty output");
-    await removeSession(storageRoot, session.sessionId);
     return { output: "", detailHash: "", sessionId: session.sessionId };
   }
 
@@ -75,9 +74,6 @@ async function runBuiltinWithMessages(
     session.startedAtMs,
   );
 
-  // Clean up session jsonl
-  await removeSession(storageRoot, session.sessionId);
-
   return { output: loopResult.finalText, detailHash, sessionId: session.sessionId };
 }
 

packages/workflow-agent-builtin/src/prompt.ts

@@ -59,6 +59,16 @@ export function buildBuiltinMessages(ctx: AgentContext): ChatMessage[] {
   }
   systemParts.push(rolePrompt);
 
+  systemParts.push(
+    "",
+    "## Workflow",
+    "",
+    "You have tools available (read_file, write_file, run_command). " +
+      "Use them to complete your task — read files, run commands, make changes as needed. " +
+      "When you are done, output your final response with the YAML frontmatter block as specified above. " +
+      "Do NOT output the frontmatter until you have completed all necessary work.",
+  );
+
   const messages: ChatMessage[] = [{ role: "system", content: systemParts.join("\n") }];
 
   const roleVisitIndices: number[] = [];