fix: restore model activity reasoning stream

frontend/src/store/sessionStore.ts

@@ -9,6 +9,7 @@ interface Entry {
 interface SessionState {
   pendingWidget: WidgetDescriptor | null;
   transcript: Entry[];
+  activity: Entry[];
   toasts: { level: string; text: string }[];
   stepMessages: { level: string; text: string }[];
   lastUserEntry: { kind: "input" | "choice"; text: string } | null;
@@ -31,6 +32,7 @@ interface SessionState {
 const empty = {
   pendingWidget: null,
   transcript: [] as Entry[],
+  activity: [] as Entry[],
   toasts: [] as { level: string; text: string }[],
   stepMessages: [] as { level: string; text: string }[],
   lastUserEntry: null as { kind: "input" | "choice"; text: string } | null,
@@ -63,6 +65,13 @@ export const useSessionStore = create<SessionState>((set) => ({
         // Streamed text means the turn is alive (also covers reattaching mid-turn).
         return { transcript: t, agentActive: true };
       }
+      if (e.type === "activity_delta") {
+        const activity = [...st.activity];
+        const last = activity.at(-1);
+        if (last) activity[activity.length - 1] = { role: "assistant", text: last.text + e.text };
+        else activity.push({ role: "assistant", text: e.text });
+        return { activity, agentActive: true };
+      }
       if (e.type === "info") {
         const stepMessages = [...st.stepMessages, { level: e.level ?? "info", text: e.text }];
         // An error during the active phase marks that phase red until the next gate.