feat(orchestrator): create_skill action + wire per-project watchers (§14.3)

- domain: OrchestratorCommand::CreateSkill with optional scope field
  (defaults to Project, case-insensitive, UnknownScope on bad value)
- application: dispatch CreateSkill through the CreateSkill use case
- app-tauri: build OrchestratorService and start/stop per-project request
  watchers on open/create/close_project (ensure/stop_orchestrator_watch)
- tests: domain validation, service dispatch, infra watcher e2e,
  app-tauri wiring lifecycle (idempotent, isolated, stop)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/features/layout/LayoutGrid.test.tsx

@@ -9,7 +9,7 @@
  * bailed.
  */
 import { describe, it, expect } from "vitest";
-import { render, screen, waitFor } from "@testing-library/react";
+import { render, screen, waitFor, fireEvent } from "@testing-library/react";
 
 import type { Gateways } from "@/ports";
 import { MockLayoutGateway, MockTerminalGateway } from "@/adapters/mock";
@@ -90,6 +90,72 @@ describe("LayoutGrid (with MockLayoutGateway)", () => {
     });
   });
 
+  it("closing a cell removes THAT cell and keeps its sibling (closes the right terminal)", async () => {
+    const layout = new MockLayoutGateway();
+    const initial = await layout.loadLayout("p1");
+    const a = leaves(initial)[0].id;
+    // Split A → container c with children [A (index 0), b (index 1)].
+    await layout.mutateLayout("p1", {
+      type: "split",
+      target: a,
+      direction: "row",
+      newLeaf: "b",
+      container: "c",
+    });
+
+    renderGrid(layout);
+    await waitFor(() =>
+      expect(screen.getAllByTestId("layout-leaf")).toHaveLength(2),
+    );
+
+    // Click the close button on cell `b` — `b` must disappear, `a` must remain.
+    fireEvent.click(screen.getByLabelText("close b"));
+
+    await waitFor(() =>
+      expect(screen.getAllByTestId("layout-leaf")).toHaveLength(1),
+    );
+    expect(
+      screen.getByTestId("layout-leaf").getAttribute("data-node-id"),
+    ).toBe(a);
+  });
+
+  it("closing the other cell keeps the opposite sibling", async () => {
+    const layout = new MockLayoutGateway();
+    const initial = await layout.loadLayout("p1");
+    const a = leaves(initial)[0].id;
+    await layout.mutateLayout("p1", {
+      type: "split",
+      target: a,
+      direction: "row",
+      newLeaf: "b",
+      container: "c",
+    });
+
+    renderGrid(layout);
+    await waitFor(() =>
+      expect(screen.getAllByTestId("layout-leaf")).toHaveLength(2),
+    );
+
+    // Closing `a` (index 0) must keep `b` (index 1).
+    fireEvent.click(screen.getByLabelText(`close ${a}`));
+
+    await waitFor(() =>
+      expect(screen.getAllByTestId("layout-leaf")).toHaveLength(1),
+    );
+    expect(
+      screen.getByTestId("layout-leaf").getAttribute("data-node-id"),
+    ).toBe("b");
+  });
+
+  it("the sole root cell has no close button (cannot be closed)", async () => {
+    const layout = new MockLayoutGateway();
+    renderGrid(layout);
+    await waitFor(() =>
+      expect(screen.getAllByTestId("layout-leaf")).toHaveLength(1),
+    );
+    expect(screen.queryByLabelText(/^close /)).toBeNull();
+  });
+
   it("renders the loading/empty container when the layout gateway is absent", () => {
     // A DI subset without the layout gateway → useLayout is defensive (null).
     const gateways = { terminal: new MockTerminalGateway() } as unknown as Gateways;