feat(lanes): offer a create/update readme run when adding a lane

An optional checkbox in the Add-lane modal starts the new lane's terminal
run right after creation with `/sg-all:sg-md-writing` as its first message,
so the lane opens already writing or refreshing its CLAUDE.md/AGENTS.md set.
Claude lanes only — codex has no slash commands.

client/src/components/lanes/__tests__/AddLaneModal.test.tsx

@@ -30,6 +30,7 @@ vi.mock("../../../lib/api", () => ({
       agentsInstall: vi.fn(),
       mcpSync: vi.fn(),
     },
+    run: { start: vi.fn() },
   },
 }));
 
@@ -398,6 +399,42 @@ describe("AddLaneModal — folder browse", () => {
   });
 });
 
+describe("AddLaneModal — create/update readme", () => {
+  it("opens the new lane's terminal on the docs skill only when the box is ticked", async () => {
+    vi.mocked(api.run.start)
+      .mockReset()
+      .mockResolvedValue({} as never);
+    vi.mocked(api.lanes.ensure).mockResolvedValue({
+      lane: laneFixture({ status: "idle", cwd: "/Users/tester/projects/repo" }),
+      created: true,
+    });
+    renderModal();
+    const user = userEvent.setup();
+
+    await user.click(screen.getByRole("button", { name: "Repo" }));
+    const repoField = screen.getByLabelText("Directory");
+    await focusField(user, repoField);
+    await user.type(repoField, "/Users/tester/projects/repo");
+    await user.click(screen.getByLabelText("Create/update readme file"));
+    await user.click(screen.getByRole("button", { name: "Add lane" }));
+
+    await waitFor(() =>
+      expect(api.run.start).toHaveBeenCalledWith({
+        laneId: 9,
+        cwd: "/Users/tester/projects/repo",
+        initialPrompt: "/sg-all:sg-md-writing",
+      })
+    );
+  });
+
+  it("is hidden for a codex lane, which has no slash commands", async () => {
+    renderModal();
+    const user = userEvent.setup();
+    await user.click(screen.getByRole("button", { name: "Codex" }));
+    expect(screen.queryByLabelText("Create/update readme file")).toBeNull();
+  });
+});
+
 describe("AddLaneModal — pipeline template", () => {
   it("offers every template the server reports and sends the chosen one when adopting a repo", async () => {
     // Creation is the ONLY point the UI can set a template, so a lane born on