feat(nexus): add CmdKButton shim for layout overhaul (phase 8)

Top-strip button that renders the ⌘K glyph and opens the existing
CommandPalette by dispatching a synthetic Meta+K keydown on document,
which CommandPalette.tsx already listens for at its useEffect
(lines 42-51). This is explicitly a Phase 8 shim; Phase 14 of
docs/specs/2026-04-11-nexus-layout-overhaul.md replaces it with a
proper command-palette context when globalizing the palette's
search index.

Part of Phase 8 of the Nexus layout overhaul (task 3 of 7).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/components/frame/CmdKButton.test.tsx

@@ -0,0 +1,62 @@
+// @vitest-environment jsdom
+
+import { act } from "react";
+import { createRoot } from "react-dom/client";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { CmdKButton } from "./CmdKButton";
+
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+(globalThis as any).IS_REACT_ACT_ENVIRONMENT = true;
+
+describe("CmdKButton", () => {
+  let container: HTMLDivElement;
+  let root: ReturnType<typeof createRoot> | null = null;
+
+  beforeEach(() => {
+    container = document.createElement("div");
+    document.body.appendChild(container);
+    root = null;
+  });
+
+  afterEach(() => {
+    if (root) {
+      act(() => { root!.unmount(); });
+      root = null;
+    }
+    if (container.parentNode) container.remove();
+  });
+
+  function renderButton() {
+    root = createRoot(container);
+    act(() => {
+      root!.render(<CmdKButton />);
+    });
+    return {
+      getButton: () => container.querySelector("button[aria-label='Open command palette']") as HTMLButtonElement,
+      getKbd: () => container.querySelector("kbd")?.textContent?.trim(),
+    };
+  }
+
+  it("renders a button with the ⌘K kbd glyph", () => {
+    const { getButton, getKbd } = renderButton();
+    expect(getButton()).not.toBeNull();
+    expect(getKbd()).toBe("⌘K");
+  });
+
+  it("dispatches a Meta+K keydown on document when clicked", () => {
+    const listener = vi.fn();
+    document.addEventListener("keydown", listener);
+
+    const { getButton } = renderButton();
+    act(() => {
+      getButton().click();
+    });
+
+    expect(listener).toHaveBeenCalledTimes(1);
+    const event = listener.mock.calls[0]![0] as KeyboardEvent;
+    expect(event.key).toBe("k");
+    expect(event.metaKey).toBe(true);
+
+    document.removeEventListener("keydown", listener);
+  });
+});

ui/src/components/frame/CmdKButton.tsx

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+/**
+ * CmdKButton — Phase 8 shim for the top-strip command palette trigger.
+ *
+ * Renders the ⌘K keyboard glyph and, when clicked, dispatches a synthetic
+ * Meta+K keydown event on `document`. The existing CommandPalette component
+ * in ui/src/components/CommandPalette.tsx installs a document-level keydown
+ * listener for Meta+K (see its useEffect at lines 42-51) and opens itself
+ * when that key is pressed, so the synthetic event reaches it without
+ * needing a refactor to share state.
+ *
+ * Phase 14 of docs/specs/2026-04-11-nexus-layout-overhaul.md replaces this
+ * shim with a proper command-palette context and globalizes the palette's
+ * search index. This file will either be deleted or gutted at that point.
+ */
+export function CmdKButton() {
+  const handleClick = () => {
+    const event = new KeyboardEvent("keydown", {
+      key: "k",
+      metaKey: true,
+      bubbles: true,
+      cancelable: true,
+    });
+    document.dispatchEvent(event);
+  };
+
+  return (
+    <button
+      type="button"
+      onClick={handleClick}
+      aria-label="Open command palette"
+      title="Open command palette (⌘K)"
+      className="inline-flex h-8 items-center gap-2 rounded-[4px] border border-border bg-card px-2 text-[12px] font-medium text-muted-foreground transition-colors hover:text-primary hover:border-primary focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-primary focus-visible:ring-offset-2 focus-visible:ring-offset-background"
+    >
+      <kbd className="font-mono text-[12px]">⌘K</kbd>
+    </button>
+  );
+}