fix(auto-reply): skip replay when retained window is assistant-only

Codex follow-up review on #70898: the `startIdx < kept.length - 1`
guard still wrote an assistant-first transcript when every turn inside
the retained window was an assistant (e.g. a tail of consecutive
assistant messages). That re-creates the exact role-ordering hazard the
reset path is recovering from.

Advance through the full window and, if no user turn remains, return 0
without touching the target transcript. Missing a few turns of DM
continuity is strictly better than re-triggering the conflict.

Locked in with a new assertion on the existing unit test.

Made-with: Cursor

src/auto-reply/reply/session-transcript-replay.test.ts

@@ -47,6 +47,15 @@ describe("replayRecentUserAssistantMessages", () => {
       expect(["user", "assistant"]).toContain(r.message.role);
     }
     expect(await call(path.join(root, "missing.jsonl"), path.join(root, "out.jsonl"))).toBe(0);
+
+    const assistantSource = path.join(root, "all-assistant.jsonl");
+    const assistantTarget = path.join(root, "all-assistant-out.jsonl");
+    const onlyAssistants = Array.from({ length: 3 }, () =>
+      j({ message: { role: "assistant", content: "x" } }),
+    ).join("");
+    await fs.writeFile(assistantSource, onlyAssistants, "utf8");
+    expect(await call(assistantSource, assistantTarget)).toBe(0);
+    await expect(fs.stat(assistantTarget)).rejects.toThrow();
   });
 
   it("skips header for pre-existing targets and aligns the tail to a user turn", async () => {

src/auto-reply/reply/session-transcript-replay.ts

@@ -47,9 +47,14 @@ export async function replayRecentUserAssistantMessages(params: {
       return 0;
     }
     let startIdx = Math.max(0, kept.length - max);
-    while (startIdx < kept.length - 1 && kept[startIdx].role === "assistant") {
+    while (startIdx < kept.length && kept[startIdx].role === "assistant") {
       startIdx += 1;
     }
+    if (startIdx === kept.length) {
+      // Retained window is assistant-only; replaying would re-create the same
+      // role-ordering hazard this reset path is recovering from.
+      return 0;
+    }
     const tail = kept.slice(startIdx).map((entry) => entry.line);
     if (!fs.existsSync(params.targetTranscript)) {
       await fsp.mkdir(path.dirname(params.targetTranscript), { recursive: true });