Allow failed agent turns to be forked (#2063)

* fix(chat): allow failed turns to be forked

Failed turns can end in Paseo-generated timeline items without provider
message IDs. Anchor forks to canonical timeline positions while retaining
legacy boundaries for compatibility.

Place the fork summary before the new prompt and delimit it as structured
chat history.

* fix(chat): tighten fork boundary handling

packages/app/src/timeline/session-stream-reducers.test.ts

@@ -187,6 +187,25 @@ const baseStreamInput: ProcessAgentStreamEventInput = {
 // ---------------------------------------------------------------------------
 
 describe("processTimelineResponse", () => {
+  it("preserves the canonical end cursor on a projected assistant message", () => {
+    const result = processTimelineResponse({
+      ...baseTimelineInput,
+      payload: {
+        ...baseTimelineInput.payload,
+        epoch: "timeline-1",
+        entries: [makeTimelineEntry(40, "[System Error] failed", "assistant_message", 42)],
+      },
+    });
+
+    expect(result.tail).toEqual([
+      expect.objectContaining({
+        kind: "assistant_message",
+        text: "[System Error] failed",
+        timelineCursor: { epoch: "timeline-1", seq: 42 },
+      }),
+    ]);
+  });
+
   it("returns error path when payload.error is set", () => {
     const result = processTimelineResponse({
       ...baseTimelineInput,
@@ -892,7 +911,12 @@ describe("processTimelineResponse", () => {
   });
 
   it("merges assistant chunks across the older-page prepend boundary", () => {
-    const currentTail = [makeAssistantItem("newer chunk", "assistant-newer")];
+    const currentTail = [
+      {
+        ...makeAssistantItem("newer chunk", "assistant-newer"),
+        timelineCursor: { epoch: "epoch-1", seq: 3 },
+      },
+    ];
     const existingCursor: TimelineCursor = {
       epoch: "epoch-1",
       startSeq: 3,
@@ -914,6 +938,9 @@ describe("processTimelineResponse", () => {
     });
 
     expect(getAssistantTexts(result.tail)).toEqual(["older chunk newer chunk"]);
+    expect(result.tail[0]).toEqual(
+      expect.objectContaining({ timelineCursor: { epoch: "epoch-1", seq: 3 } }),
+    );
     expect(result.cursor).toEqual({
       epoch: "epoch-1",
       startSeq: 1,
@@ -1222,6 +1249,23 @@ describe("processTimelineResponse", () => {
 // ---------------------------------------------------------------------------
 
 describe("processAgentStreamEvent", () => {
+  it("preserves the live timeline cursor on an assistant error", () => {
+    const result = processAgentStreamEvent({
+      ...baseStreamInput,
+      event: makeAssistantTimelineEvent("[System Error] failed"),
+      epoch: "timeline-1",
+      seq: 42,
+    });
+
+    expect(result.head).toEqual([
+      expect.objectContaining({
+        kind: "assistant_message",
+        text: "[System Error] failed",
+        timelineCursor: { epoch: "timeline-1", seq: 42 },
+      }),
+    ]);
+  });
+
   it("passes through non-timeline events without cursor changes", () => {
     const turnEvent: AgentStreamEventPayload = {
       type: "turn_completed",