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;

frontend/src/features/layout/LayoutGrid.tsx

@@ -113,7 +113,14 @@ interface LeafViewProps {
 }
 
 function LeafView({ id, session, agent, cwd, vm, parentSplit, projectId }: LeafViewProps) {
-  const canMerge = parentSplit !== null && parentSplit.siblings > 1;
+  // A cell can be closed only when it lives inside a (binary) split: closing it
+  // collapses the parent split, keeping the *sibling*. Splits are always binary
+  // in this model (a split wraps a leaf into a 2-child container), so the kept
+  // child is simply the other index. Guarding on `siblings === 2` keeps the
+  // operation correct (merge keeps exactly one child) and avoids ever dropping
+  // the wrong terminal. The root cell (no parent split) cannot be closed.
+  const canClose = parentSplit !== null && parentSplit.siblings === 2;
+  const siblingIndex = parentSplit ? (parentSplit.index === 0 ? 1 : 0) : 0;
   const { agent: agentGateway } = useGateways();
 
   // Load the project's agents for the dropdown.
@@ -159,7 +166,10 @@ function LeafView({ id, session, agent, cwd, vm, parentSplit, projectId }: LeafV
         style={{
           position: "absolute",
           top: 2,
-          right: 2,
+          // Clear the xterm viewport scrollbar (rendered flush against the right
+          // edge, ~15px wide). Without this offset the right-most control — the
+          // close button — sits *behind* the scrollbar and is hard to click.
+          right: 16,
           zIndex: 2,
           display: "flex",
           gap: 2,
@@ -208,16 +218,14 @@ function LeafView({ id, session, agent, cwd, vm, parentSplit, projectId }: LeafV
         >
           ⬍
         </button>
-        {canMerge && (
+        {canClose && (
           <button
             type="button"
-            title="Merge: keep this cell, drop its siblings"
-            aria-label={`merge ${id}`}
-            onClick={() =>
-              void vm.merge(parentSplit.container, parentSplit.index)
-            }
+            title="Close this terminal"
+            aria-label={`close ${id}`}
+            onClick={() => void vm.merge(parentSplit.container, siblingIndex)}
           >
-            ⤬
+            ✕
           </button>
         )}
       </div>