feat(app): add chat outline navigation (#2792)

* feat(app): add chat outline navigation

Long agent sessions need prompt navigation without coupling viewport movement to disposable server windows. Keep fetched timeline pages in client state so repeated jumps stay local, and settle prompt jumps against virtualized row measurement changes.

* fix(app): preserve chat outline timeline coverage

* fix(app): reconcile outline window submissions

* fix(app): harden outline timeline navigation

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

@@ -42,6 +42,7 @@ export interface StreamEdgeSlotProps {
 export interface StreamViewportHandle {
   scrollToBottom: (reason?: BottomAnchorLocalRequest["reason"]) => void;
   prepareForViewportChange: () => void;
+  scrollToMessage?: (itemId: string) => void;
 }
 
 export interface StreamSegmentRenderers {
@@ -69,6 +70,9 @@ export interface StreamRenderInput {
   routeBottomAnchorRequest: BottomAnchorRouteRequest | null;
   isAuthoritativeHistoryReady: boolean;
   onNearBottomChange: (value: boolean) => void;
+  // The history row under the top of the viewport, for surfaces that mark where the reader
+  // is in the transcript. Only the web viewport measures it today.
+  onReadingPositionChange?: (rowId: string | null) => void;
   onNearHistoryStart: () => boolean | Promise<boolean>;
   isLoadingOlderHistory: boolean;
   hasOlderHistory: boolean;