feat(FN-5442): add merge mode warning banner to SettingsModal

Added a merge mode warning banner to SettingsModal with styling and visibility tests.

Fusion-Task-Id: FN-5442

packages/dashboard/app/components/SettingsModal.css

@@ -2014,9 +2014,29 @@
   gap: var(--space-sm);
 }
 
+.settings-warning-banner {
+  margin-top: var(--space-sm);
+  padding: var(--space-sm) var(--space-md);
+  border: 1px solid var(--color-warning);
+  border-radius: var(--radius-md);
+  background: color-mix(in srgb, var(--color-warning) 12%, transparent);
+  color: var(--text);
+  font-size: var(--font-sm, 0.875rem);
+  line-height: 1.4;
+}
+
+.settings-warning-banner strong {
+  color: var(--color-warning);
+  margin-right: var(--space-xs);
+}
+
 @media (max-width: 768px) {
   .auth-custom-provider-item {
     flex-direction: column;
     align-items: flex-start;
   }
+
+  .settings-warning-banner {
+    padding: var(--space-sm);
+  }
 }

packages/dashboard/app/components/SettingsModal.tsx

@@ -4639,6 +4639,22 @@ export function SettingsModal({
                   <small>
                     Auto-merge runs in the task worktree by default. Switch to the legacy project-root path only if you need the pre-FN-5279 fallback; worktrunk-managed projects still defer to worktrunk.
                   </small>
+                  {(form.mergeIntegrationWorktree ?? "reuse-task-worktree") !== "reuse-task-worktree" && (
+                    <div
+                      className="settings-warning-banner"
+                      role="alert"
+                      aria-live="polite"
+                      data-testid="merge-integration-worktree-warning"
+                    >
+                      <strong>Legacy integration-branch mode.</strong>{" "}
+                      Auto-merge will run rebase, conflict resolution, and squash commits inside the
+                      project root (the user&apos;s checked-out integration-branch worktree) instead of
+                      the task worktree. Fusion assumes that directory is already on the integration
+                      branch and clean; if it isn&apos;t, merges may fail or touch the user&apos;s working
+                      tree. Reuse-task-worktree is the recommended default (FN-5279). Switch back unless
+                      you have a specific reason to opt in (FN-5348).
+                    </div>
+                  )}
                 </div>
               </>
             )}

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

@@ -396,6 +396,46 @@ describe("SettingsModal", () => {
     expect(payload.mergeIntegrationWorktree).toBe("cwd-main");
   });
 
+  it("does NOT render the warning banner when the integration worktree is reuse-task-worktree (default)", async () => {
+    renderModal({ initialSection: "merge" });
+    await waitForSettingsModalReady();
+
+    expect(screen.queryByTestId("merge-integration-worktree-warning")).toBeNull();
+  });
+
+  it("renders the warning banner when the legacy cwd-main mode is selected", async () => {
+    renderModal({ initialSection: "merge" });
+    await waitForSettingsModalReady();
+
+    await userEvent.selectOptions(screen.getByLabelText("Integration worktree"), "cwd-main");
+
+    const warning = screen.getByTestId("merge-integration-worktree-warning");
+    expect(warning).toBeInTheDocument();
+    expect(warning).toHaveAttribute("role", "alert");
+    expect(warning).toHaveTextContent("Legacy");
+    expect(warning).toHaveTextContent("FN-5348");
+  });
+
+  it("removes the warning banner when switching back to reuse-task-worktree", async () => {
+    mockFetchSettings.mockResolvedValueOnce({
+      ...defaultSettings,
+      mergeIntegrationWorktree: "cwd-main",
+    });
+    mockFetchSettingsByScope.mockResolvedValueOnce({
+      global: { ...defaultSettings, mergeIntegrationWorktree: "cwd-main" },
+      project: {},
+    });
+
+    renderModal({ initialSection: "merge" });
+    await waitForSettingsModalReady();
+
+    expect(screen.getByTestId("merge-integration-worktree-warning")).toBeInTheDocument();
+
+    await userEvent.selectOptions(screen.getByLabelText("Integration worktree"), "reuse-task-worktree");
+
+    expect(screen.queryByTestId("merge-integration-worktree-warning")).toBeNull();
+  });
+
   it("persists the legacy sibling branch rename escape hatch in worktree settings", async () => {
     renderModal();
     await waitForSettingsModalReady();