feat(FN-1151): preserve and display planning conversation history

- Persist per-turn thinking output for planning and mission interview sessions, including history serialization and recovery-safe state fields
- Add shared conversation history parsing/API types and a reusable timeline component with formatted answers and expandable AI reasoning
- Restore and render conversation history across Planning Mode, Mission Interview, and Subtask Breakdown modals during resume and live question flow
- Harden response submission with nullable-question guards and expand unit/e2e coverage for history rendering and persistence behavior

packages/dashboard/app/components/__tests__/ConversationHistory.test.tsx

@@ -0,0 +1,75 @@
+import { describe, expect, it } from "vitest";
+import { fireEvent, render, screen } from "@testing-library/react";
+import type { PlanningQuestion } from "@fusion/core";
+import { ConversationHistory } from "../ConversationHistory";
+
+const baseQuestion: PlanningQuestion = {
+  id: "q-scope",
+  type: "single_select",
+  question: "What is the project scope?",
+  options: [
+    { id: "small", label: "Small" },
+    { id: "medium", label: "Medium" },
+  ],
+};
+
+describe("ConversationHistory", () => {
+  it("renders question and formatted response pairs", () => {
+    render(
+      <ConversationHistory
+        entries={[
+          {
+            question: baseQuestion,
+            response: { "q-scope": "medium" },
+          },
+        ]}
+      />,
+    );
+
+    expect(screen.getByText("Q1")).toBeDefined();
+    expect(screen.getByText("What is the project scope?")).toBeDefined();
+    expect(screen.getByText("Medium")).toBeDefined();
+  });
+
+  it("shows thinking output when expanded", () => {
+    render(
+      <ConversationHistory
+        entries={[
+          {
+            question: { ...baseQuestion, id: "q1" },
+            response: { q1: "small" },
+            thinkingOutput: "Internal reasoning for first question",
+          },
+        ]}
+      />,
+    );
+
+    expect(screen.queryByText("Internal reasoning for first question")).toBeNull();
+
+    fireEvent.click(screen.getByRole("button", { name: /Show AI thinking/i }));
+
+    expect(screen.getByText("Internal reasoning for first question")).toBeDefined();
+  });
+
+  it("returns null for empty entries", () => {
+    const { container } = render(<ConversationHistory entries={[]} />);
+
+    expect(container.firstChild).toBeNull();
+  });
+
+  it("renders entries that only contain thinking output", () => {
+    render(
+      <ConversationHistory
+        entries={[
+          {
+            thinkingOutput: "Reasoning captured during subtask generation",
+          },
+        ]}
+      />,
+    );
+
+    expect(screen.getByText("AI Reasoning")).toBeDefined();
+    fireEvent.click(screen.getByRole("button", { name: /Show AI reasoning/i }));
+    expect(screen.getByText("Reasoning captured during subtask generation")).toBeDefined();
+  });
+});

packages/dashboard/app/components/__tests__/MissionInterviewModal.test.tsx

@@ -13,6 +13,7 @@ vi.mock("../../api", () => ({
   createMissionFromInterview: vi.fn(),
   connectMissionInterviewStream: vi.fn(),
   fetchAiSession: vi.fn(),
+  parseConversationHistory: vi.fn(),
 }));
 
 vi.mock("../../hooks/modalPersistence", () => ({
@@ -27,6 +28,7 @@ const mockCancelMissionInterview = vi.mocked(api.cancelMissionInterview);
 const mockCreateMissionFromInterview = vi.mocked(api.createMissionFromInterview);
 const mockConnectMissionInterviewStream = vi.mocked(api.connectMissionInterviewStream);
 const mockFetchAiSession = vi.mocked(api.fetchAiSession);
+const mockParseConversationHistory = vi.mocked(api.parseConversationHistory);
 const mockGetMissionGoal = vi.mocked(modalPersistence.getMissionGoal);
 
 const sampleQuestionSingle: PlanningQuestion = {
@@ -84,6 +86,15 @@ describe("MissionInterviewModal", () => {
     mockCancelMissionInterview.mockResolvedValue(undefined);
     mockCreateMissionFromInterview.mockResolvedValue({ id: "MS-001", title: "Created mission" } as any);
     mockFetchAiSession.mockResolvedValue(null);
+    mockParseConversationHistory.mockImplementation((raw: string) => {
+      if (!raw) return [];
+      try {
+        const parsed = JSON.parse(raw);
+        return Array.isArray(parsed) ? parsed : [];
+      } catch {
+        return [];
+      }
+    });
     mockGetMissionGoal.mockReturnValue("");
 
     mockConnectMissionInterviewStream.mockImplementation((_sessionId, _projectId, handlers) => {