feat(FN-4069): add direct merge commit routing to merger

Adds direct merge commit routing to the merger, allowing non-squash merges to bypass the squash-audit path when configured. The feature includes new `mergeCommitStrategy` settings, updated dashboard UI, expanded merger lifecycle tests, and documentation.

Fusion-Task-Id: FN-4069

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

@@ -177,6 +177,7 @@ const defaultSettings = {
   overlapIgnorePaths: [],
   autoMerge: true,
   mergeStrategy: "direct",
+  directMergeCommitStrategy: "auto",
   pushAfterMerge: false,
   pushRemote: "origin",
   verificationFixRetries: 2,
@@ -248,6 +249,17 @@ describe("SettingsModal", () => {
     expect(container.querySelectorAll(".settings-section-heading").length).toBeGreaterThan(0);
   });
 
+  it("shows direct merge commit routing only for direct merges", async () => {
+    renderModal();
+    await waitForSettingsModalReady();
+
+    await userEvent.click(screen.getByRole("button", { name: /^Merge$/ }));
+    expect(screen.getByLabelText("Direct merge commit routing")).toHaveValue("auto");
+
+    await userEvent.selectOptions(screen.getByLabelText("Auto-completion mode"), "pull-request");
+    expect(screen.queryByLabelText("Direct merge commit routing")).not.toBeInTheDocument();
+  });
+
   beforeEach(() => {
     vi.clearAllMocks();
     clearPluginUiSlotsCache();