Fork assistant turns into new drafts (#1788)

* feat(chat): fork assistant turns into new drafts

Builds chat-history attachments on the daemon and routes them into draft composers so users can continue an assistant turn in an existing workspace tab or New Workspace.

* fix(chat): clean up fork draft handling

* fix(chat): address fork draft review feedback

* fix(server): build fork context from bounded timeline

* fix(app): restore assistant turn footer in streams

* fix(app): preserve fork draft setup

* fix(app): tighten assistant fork footers

* fix(app): lock active fork draft handoff

* fix(app): preserve attachment-only draft retries

* fix(app): unify composer attachment scopes

* fix(app): align chat history attachment labels

* fix(app): centralize attachment pill content

* fix(app): trim fork context for workspace naming

packages/app/src/agent-stream/layout.ts

@@ -68,18 +68,48 @@ function createTurnFooterHost(input: {
   };
 }
 
+function findLatestAssistantIndexInTurn(input: {
+  strategy: StreamStrategy;
+  items: StreamItem[];
+  startIndex: number;
+}): number | null {
+  for (
+    let index = input.startIndex;
+    index >= 0 && index < input.items.length;
+    index = input.strategy.getNeighborIndex(index, "above")
+  ) {
+    const item = input.items[index];
+    if (!item || item.kind === "user_message") {
+      return null;
+    }
+    if (item.kind === "assistant_message") {
+      return index;
+    }
+  }
+  return null;
+}
+
 function resolveAuxiliaryTurnFooter(input: StreamLayoutInput): TurnFooterHost | null {
   if (input.agentStatus === "running") {
     return null;
   }
 
   const footerItems = input.liveHead.length > 0 ? input.liveHead : input.history;
-  const startIndex = input.strategy.getLatestItemIndex(footerItems);
-  if (startIndex === null) {
+  const latestIndex = input.strategy.getLatestItemIndex(footerItems);
+  if (latestIndex === null) {
     return null;
   }
 
-  const item = footerItems[startIndex];
+  const assistantIndex = findLatestAssistantIndexInTurn({
+    strategy: input.strategy,
+    items: footerItems,
+    startIndex: latestIndex,
+  });
+  if (assistantIndex === null) {
+    return null;
+  }
+
+  const item = footerItems[assistantIndex];
   if (!item || item.kind !== "assistant_message") {
     return null;
   }
@@ -87,26 +117,79 @@ function resolveAuxiliaryTurnFooter(input: StreamLayoutInput): TurnFooterHost |
   return createTurnFooterHost({
     item,
     items: footerItems,
-    index: startIndex,
+    index: assistantIndex,
     timingByAssistantId: input.timingByAssistantId,
   });
 }
 
+function findTurnEndIndexInSegment(input: {
+  strategy: StreamStrategy;
+  items: StreamItem[];
+  startIndex: number;
+}): number {
+  let endIndex = input.startIndex;
+  for (
+    let index = input.strategy.getNeighborIndex(input.startIndex, "below");
+    index >= 0 && index < input.items.length;
+    index = input.strategy.getNeighborIndex(index, "below")
+  ) {
+    const item = input.items[index];
+    if (!item || item.kind === "user_message") {
+      break;
+    }
+    endIndex = index;
+  }
+  return endIndex;
+}
+
 function shouldRenderCompletedFooter(input: {
+  strategy: StreamStrategy;
+  items: StreamItem[];
+  index: number;
   item: StreamItem;
   belowItem: StreamItem | null;
   agentStatus: string;
   auxiliaryTurnFooter: TurnFooterHost | null;
 }): boolean {
-  return (
-    input.item.kind === "assistant_message" &&
-    input.auxiliaryTurnFooter?.itemId !== input.item.id &&
-    (input.belowItem?.kind === "user_message" ||
-      (input.belowItem === null && input.agentStatus !== "running"))
-  );
+  if (
+    input.item.kind !== "assistant_message" ||
+    input.auxiliaryTurnFooter?.itemId === input.item.id
+  ) {
+    return false;
+  }
+
+  if (
+    input.belowItem?.kind === "user_message" ||
+    (input.belowItem === null && input.agentStatus !== "running")
+  ) {
+    return true;
+  }
+
+  if (!isToolSequenceItem(input.belowItem)) {
+    return false;
+  }
+
+  const sameSegmentBelowItem = input.strategy.getNeighborItem(input.items, input.index, "below");
+  if (sameSegmentBelowItem?.id !== input.belowItem.id) {
+    return false;
+  }
+
+  const turnEndIndex = findTurnEndIndexInSegment({
+    strategy: input.strategy,
+    items: input.items,
+    startIndex: input.index,
+  });
+  const assistantIndex = findLatestAssistantIndexInTurn({
+    strategy: input.strategy,
+    items: input.items,
+    startIndex: turnEndIndex,
+  });
+  return assistantIndex === input.index;
 }
 
-function isToolSequenceItem(item: StreamItem | null): boolean {
+function isToolSequenceItem(
+  item: StreamItem | null,
+): item is Extract<StreamItem, { kind: "tool_call" | "thought" | "todo_list" }> {
   return item?.kind === "tool_call" || item?.kind === "thought" || item?.kind === "todo_list";
 }
 
@@ -175,6 +258,9 @@ function layoutSegment(input: LayoutSegmentInput): StreamLayoutItem[] {
       belowItem,
     });
     const completedFooter = shouldRenderCompletedFooter({
+      strategy: input.strategy,
+      items: input.items,
+      index,
       item,
       belowItem,
       agentStatus: input.agentStatus,