FN-5677: add per-task auto-merge override controls

Add dashboard and API support to set or clear per-task auto-merge overrides during review.

- add TaskReviewTab UI controls and styles for selecting or clearing an auto-merge override
- wire legacy dashboard API request handling for task auto-merge override updates
- update task workflow routes to persist override operations
- add dashboard and route tests covering set/clear override behavior

Files changed:
 packages/dashboard/app/__tests__/api-tasks.test.ts | 26 +++++++++
 packages/dashboard/app/api/legacy.ts               |  1 +
 packages/dashboard/app/components/TaskReviewTab.css     | 17 ++++++
 packages/dashboard/app/components/TaskReviewTab.tsx     | 55 +++++++++++++++++-
 packages/dashboard/app/components/__tests__/TaskReviewTab.test.tsx    | 48 ++++++++++++++++
 packages/dashboard/src/__tests__/routes-tasks-ops.test.ts         | 65 ++++++++++++++++++++++
 packages/dashboard/src/routes/register-task-workflow-routes.ts    |  7 ++-
 7 files changed, 216 insertions(+), 3 deletions(-)

Fusion-Task-Id: FN-5677

Fusion-Task-Lineage: 4503e605-1190-4a32-8736-dd11eb2756b8

packages/dashboard/src/__tests__/routes-tasks-ops.test.ts

@@ -2953,6 +2953,71 @@ describe("PATCH /tasks/:id", () => {
     const updateArg = (store.updateTask as ReturnType<typeof vi.fn>).mock.calls[0][1];
     expect(updateArg).not.toHaveProperty("executionMode");
   });
+
+  it("forwards autoMerge to store.updateTask", async () => {
+    (store.updateTask as ReturnType<typeof vi.fn>).mockResolvedValue({
+      ...FAKE_TASK_DETAIL,
+      autoMerge: true,
+    });
+
+    const res = await REQUEST(buildApp(), "PATCH", "/api/tasks/KB-001", JSON.stringify({
+      autoMerge: true,
+    }), {
+      "Content-Type": "application/json",
+    });
+
+    expect(res.status).toBe(200);
+    expect(store.updateTask).toHaveBeenCalledWith("KB-001", {
+      autoMerge: true,
+    });
+  });
+
+  it("accepts null to clear autoMerge via PATCH", async () => {
+    (store.updateTask as ReturnType<typeof vi.fn>).mockResolvedValue({
+      ...FAKE_TASK_DETAIL,
+      autoMerge: undefined,
+    });
+
+    const res = await REQUEST(buildApp(), "PATCH", "/api/tasks/KB-001", JSON.stringify({
+      autoMerge: null,
+    }), {
+      "Content-Type": "application/json",
+    });
+
+    expect(res.status).toBe(200);
+    expect(store.updateTask).toHaveBeenCalledWith("KB-001", {
+      autoMerge: undefined,
+    });
+  });
+
+  it("returns 400 for invalid autoMerge value via PATCH", async () => {
+    const res = await REQUEST(buildApp(), "PATCH", "/api/tasks/KB-001", JSON.stringify({
+      autoMerge: "yes",
+    }), {
+      "Content-Type": "application/json",
+    });
+
+    expect(res.status).toBe(400);
+    expect(res.body.error).toContain("autoMerge must be a boolean");
+  });
+
+  it("omission does not overwrite autoMerge via PATCH", async () => {
+    const existingTask = {
+      ...FAKE_TASK_DETAIL,
+      autoMerge: false,
+    };
+    (store.updateTask as ReturnType<typeof vi.fn>).mockResolvedValue(existingTask);
+
+    const res = await REQUEST(buildApp(), "PATCH", "/api/tasks/KB-001", JSON.stringify({
+      title: "Updated Title",
+    }), {
+      "Content-Type": "application/json",
+    });
+
+    expect(res.status).toBe(200);
+    const updateArg = (store.updateTask as ReturnType<typeof vi.fn>).mock.calls[0][1];
+    expect(updateArg).not.toHaveProperty("autoMerge");
+  });
 });