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.ts

@@ -510,6 +510,7 @@ function mergePrependedCanonicalTail(olderTail: StreamItem[], currentTail: Strea
       ...olderLast,
       text: `${olderLast.text}${currentFirst.text}`,
       timestamp: currentFirst.timestamp,
+      ...(currentFirst.timelineCursor ? { timelineCursor: currentFirst.timelineCursor } : {}),
     },
     ...currentTail.slice(1),
   ];
@@ -519,8 +520,9 @@ function replaceLiveAssistantWithProjectedText(params: {
   head: StreamItem[];
   event: AgentStreamEventPayload;
   timestamp: Date;
+  timelineCursor: { epoch: string; seq: number };
 }): StreamItem[] | null {
-  const { head, event, timestamp } = params;
+  const { head, event, timestamp, timelineCursor } = params;
   if (event.type !== "timeline" || event.item.type !== "assistant_message") {
     return null;
   }
@@ -537,6 +539,7 @@ function replaceLiveAssistantWithProjectedText(params: {
     ...current,
     text: event.item.text,
     timestamp,
+    timelineCursor,
   };
   return next;
 }
@@ -575,19 +578,22 @@ function applyTimelineIncrementalPath(args: {
   if (acceptedUnits.length > 0) {
     if (payload.direction === "before") {
       const olderTail = hydrateStreamState(
-        acceptedUnits.map(({ event, timestamp }) => ({
+        acceptedUnits.map(({ event, timestamp, seqEnd }) => ({
           event,
           timestamp,
+          timelineCursor: { epoch: payload.epoch, seq: seqEnd },
         })),
         { source: "canonical" },
       );
       nextTail = mergePrependedCanonicalTail(olderTail, currentTail);
     } else if (currentHead.length > 0) {
-      for (const { event, timestamp } of acceptedUnits) {
+      for (const { event, timestamp, seqEnd } of acceptedUnits) {
+        const timelineCursor = { epoch: payload.epoch, seq: seqEnd };
         const replacedHead = replaceLiveAssistantWithProjectedText({
           head: nextHead,
           event,
           timestamp,
+          timelineCursor,
         });
         if (replacedHead) {
           nextHead = replacedHead;
@@ -599,15 +605,17 @@ function applyTimelineIncrementalPath(args: {
           event,
           timestamp,
           source: "canonical",
+          timelineCursor,
         });
         nextTail = applied.tail;
         nextHead = applied.head;
       }
     } else {
       nextTail = acceptedUnits.reduce<StreamItem[]>(
-        (state, { event, timestamp }) =>
+        (state, { event, timestamp, seqEnd }) =>
           reduceStreamUpdate(state, event, timestamp, {
             source: "canonical",
+            timelineCursor: { epoch: payload.epoch, seq: seqEnd },
           }),
         currentTail,
       );
@@ -681,8 +689,16 @@ export function processTimelineResponse(
 
   const toHydratedEvents = (
     units: TimelineUnit[],
-  ): Array<{ event: AgentStreamEventPayload; timestamp: Date }> =>
-    units.map(({ event, timestamp }) => ({ event, timestamp }));
+  ): Array<{
+    event: AgentStreamEventPayload;
+    timestamp: Date;
+    timelineCursor: { epoch: string; seq: number };
+  }> =>
+    units.map(({ event, timestamp, seqEnd }) => ({
+      event,
+      timestamp,
+      timelineCursor: { epoch: payload.epoch, seq: seqEnd },
+    }));
 
   // ------------------------------------------------------------------
   // Derive bootstrap policy (replace vs incremental)
@@ -934,6 +950,10 @@ export function processAgentStreamEvent(
     input;
 
   const sequencing = processTimelineSequencingGate({ event, seq, epoch, currentCursor });
+  const timelineCursor =
+    event.type === "timeline" && seq !== undefined && epoch !== undefined
+      ? { epoch, seq }
+      : undefined;
 
   // ------------------------------------------------------------------
   // Apply stream event to tail/head
@@ -945,6 +965,7 @@ export function processAgentStreamEvent(
         event,
         timestamp,
         source: "live",
+        timelineCursor,
       })
     : {
         tail: currentTail,