fix(engine): abort signal races gen.next() to fix flaky kill test (#209)

Root cause: executeThread awaited gen.next() without racing against
the abort signal. When a workflow bundle awaited a long setTimeout
between yields, the engine could not respond to kill until the
Promise resolved — causing the kill test to flake when the thread
completed before kill arrived.

Fix: Promise.race gen.next() with an abort listener so kill takes
effect immediately, even mid-yield. Also move the bundle's delay
to after the first yield (between planner and coder) to ensure the
thread is killable while running.

Closes #209

packages/cli-workflow/__tests__/thread-cli.test.ts

@@ -70,10 +70,10 @@ const cliEntryPath = fileURLToPath(new URL("../src/cli.ts", import.meta.url));
 const abortablePlannerBundleSource = `${threadFixtureDescriptor}
 ${wfPutImport}
 export const run = async function* (input, options) {
-  await new Promise((r) => setTimeout(r, 600));
   const cas = options.cas;
   let h = await putContentMerkleNode(cas, "plan");
   yield { role: "planner", contentHash: h, meta: { plan: input.prompt }, refs: [h] };
+  await new Promise((r) => setTimeout(r, 10000));
   h = await putContentMerkleNode(cas, "code");
   yield { role: "coder", contentHash: h, meta: { diff: "y" }, refs: [h] };
   return { returnCode: 0, summary: "done" };

packages/workflow-execute/src/engine/engine.ts

@@ -299,7 +299,37 @@ async function driveWorkflowGenerator(params: {
       });
     }
 
-    const iterResult = await gen.next();
+    const iterResult = await Promise.race([
+      gen.next(),
+      new Promise<never>((_, reject) => {
+        if (executeOptions.signal.aborted) {
+          reject(new DOMException("The operation was aborted", "AbortError"));
+          return;
+        }
+        executeOptions.signal.addEventListener(
+          "abort",
+          () => reject(new DOMException("The operation was aborted", "AbortError")),
+          { once: true },
+        );
+      }),
+    ]).catch((e) => {
+      if (e instanceof DOMException && e.name === "AbortError") {
+        return { done: true as const, value: { returnCode: 130, summary: "thread aborted" } };
+      }
+      throw e;
+    });
+
+    if (executeOptions.signal.aborted || (iterResult.done && iterResult.value.returnCode === 130)) {
+      return await finalizeAbortedThread({
+        cas,
+        bundleDir,
+        threadId,
+        startHash,
+        chain,
+        logger,
+        abortLogTag: "H4KQ7RW3",
+      });
+    }
 
     if (iterResult.done) {
       logger("F3HN8QKP", `thread ${threadId} generator finished`);