feat(FN-4815): complete Step 1 — add duplicate scenario fixture

Fusion-Task-Id: FN-4815
Fusion-Task-Lineage: dc29e38d-5914-40c2-b9f7-3504606d91a3

packages/engine/src/__tests__/fixtures/triage-duplicate-scenario.ts

@@ -0,0 +1,61 @@
+import { vi } from "vitest";
+import type { Settings, Task, TaskStore } from "@fusion/core";
+
+/**
+ * FN-4726 / FN-4734 / FN-4741 documented a triple-duplicate triage incident where
+ * semantically identical work was repeatedly filed after the original fix was already
+ * completed. FN-4774 fixed that failure mode by adding fn_task_search to triage tools
+ * and explicit prompt guidance to search done/archived tasks before creating new work.
+ * FN-4815 adds a regression backstop so this scenario remains reusable across future
+ * triage reliability tests.
+ */
+export function createTriageDuplicateScenario() {
+  const doneTask: Task = {
+    id: "FN-DUP-DONE",
+    title: "Fix mobile nav overlap on iOS Safari",
+    description:
+      "Resolve iOS Safari viewport overlap where the mobile nav obscures the bottom row.",
+    column: "done",
+    dependencies: [],
+    steps: [],
+    currentStep: 0,
+    log: [],
+    createdAt: "2026-01-01T00:00:00.000Z",
+    updatedAt: "2026-01-01T00:00:00.000Z",
+  };
+
+  const incomingRequest = {
+    title: "Mobile nav still overlapping on iOS Safari",
+    description:
+      "Users report the bottom action row is covered by mobile nav on iPhone Safari. Check prior fixes before creating a new task.",
+  };
+
+  const searchQuery = "mobile nav overlap iOS Safari bottom action row";
+  const expectedMatchIds = [doneTask.id];
+
+  function buildMockStore(): TaskStore {
+    return {
+      searchTasks: vi.fn().mockResolvedValue([doneTask]),
+      listTasks: vi.fn().mockResolvedValue([]),
+      getSettings: vi.fn().mockResolvedValue({
+        maxConcurrent: 2,
+        maxWorktrees: 4,
+        pollIntervalMs: 10000,
+        autoMerge: true,
+      } as Settings),
+      logEntry: vi.fn().mockResolvedValue(undefined),
+      appendAgentLog: vi.fn().mockResolvedValue(undefined),
+      updateTask: vi.fn().mockResolvedValue(undefined),
+      on: vi.fn(),
+      emit: vi.fn(),
+    } as unknown as TaskStore;
+  }
+
+  return {
+    doneTask,
+    incomingRequest,
+    searchQuery,
+    expectedMatchIds,
+    buildMockStore,
+  };
+}