feat(studio): storyboard empty state with agent copy-prompt and skeleton preview (#2091)

The storyboard view's empty state was a single "no storyboard yet" line. Add a
copy-to-clipboard prompt box (a ready-to-paste handoff prompt carrying the
canonical STORYBOARD.md frontmatter + per-frame format) so users can hand it to
their coding agent, plus a faded skeleton of the contact-sheet grid so landing
on an empty board previews what a filled one looks like instead of a dead end.

packages/studio/src/components/storyboard/StoryboardView.tsx

@@ -1,5 +1,7 @@
-import type { ReactNode } from "react";
+import { useState, type ReactNode } from "react";
+import { Copy, Check } from "@phosphor-icons/react";
 import { useStoryboard } from "../../hooks/useStoryboard";
+import { copyTextToClipboard } from "../../utils/clipboard";
 import { Button } from "../ui/Button";
 import { StoryboardLoaded } from "./StoryboardLoaded";
 
@@ -70,15 +72,91 @@ function Message({ children, tone = "muted" }: { children: ReactNode; tone?: "mu
   );
 }
 
+function handoffPrompt(path: string): string {
+  return `Create a \`${path}\` at the project root to plan this video frame by frame.
+
+Use this format:
+
+---
+format: 1920x1080
+message: <the one-line takeaway of the video>
+arc: <the narrative shape, e.g. Problem → Solution>
+audience: <who it's for>
+---
+
+## Frame 1 — <title>
+- duration: 5s
+- transition_in: crossfade
+- status: planned
+- src: compositions/frames/01-<slug>.html
+
+<A sentence or two: what's on screen and what the narration says.>
+
+Add one \`## Frame N\` section per beat. Keep the arc tight.`;
+}
+
 function EmptyState({ path }: { path: string }) {
+  const [copied, setCopied] = useState(false);
+  const prompt = handoffPrompt(path);
+
+  const onCopy = async () => {
+    if (await copyTextToClipboard(prompt)) {
+      setCopied(true);
+      window.setTimeout(() => setCopied(false), 2000);
+    }
+  };
+
   return (
-    <div className="rounded-lg border border-dashed border-neutral-800 px-6 py-16 text-center">
-      <h2 className="text-base font-semibold text-neutral-300">No storyboard yet</h2>
-      <p className="mx-auto mt-2 max-w-md text-sm text-neutral-500">
-        Add a <code className="rounded bg-neutral-900 px-1 py-0.5 text-neutral-400">{path}</code> at
-        the project root to plan this video frame by frame. Your agent can create and iterate on it
-        for you.
-      </p>
+    <div>
+      <div className="rounded-lg border border-dashed border-neutral-800 px-6 py-10 text-center">
+        <h2 className="text-base font-semibold text-neutral-300">No storyboard yet</h2>
+        <p className="mx-auto mt-2 max-w-md text-sm text-neutral-500">
+          Add a <code className="rounded bg-neutral-900 px-1 py-0.5 text-neutral-400">{path}</code>{" "}
+          at the project root to plan this video frame by frame. Hand this prompt to your coding
+          agent to scaffold it.
+        </p>
+
+        <div className="mx-auto mt-6 max-w-2xl overflow-hidden rounded-lg border border-neutral-800 bg-neutral-900 text-left">
+          <div className="flex items-center justify-between border-b border-neutral-800 px-3 py-2">
+            <span className="font-mono text-xs text-neutral-500">Prompt for your agent</span>
+            <Button
+              size="sm"
+              variant="secondary"
+              onClick={onCopy}
+              icon={copied ? <Check size={14} /> : <Copy size={14} />}
+            >
+              {copied ? "Copied" : "Copy prompt"}
+            </Button>
+          </div>
+          <pre className="max-h-64 overflow-auto px-4 py-3 font-mono text-xs leading-relaxed text-neutral-400 whitespace-pre-wrap">
+            {prompt}
+          </pre>
+        </div>
+      </div>
+
+      <SkeletonPreview />
+    </div>
+  );
+}
+
+/** Faded placeholder of a filled board so landing here isn't a dead end —
+ *  it previews the contact-sheet layout {@link StoryboardGrid} renders. */
+function SkeletonPreview() {
+  return (
+    <div aria-hidden="true" className="mt-10 select-none opacity-40">
+      <div className="mb-4 text-center text-xs uppercase tracking-wide text-neutral-600">
+        Preview
+      </div>
+      <div className="grid gap-x-6 gap-y-8 [grid-template-columns:repeat(auto-fill,minmax(300px,1fr))]">
+        {[0, 1, 2].map((i) => (
+          <div key={i} className="rounded-lg border border-neutral-800 bg-neutral-900/40 p-3">
+            <div className="aspect-video w-full rounded bg-neutral-800/60" />
+            <div className="mt-3 h-3 w-2/3 rounded bg-neutral-800/60" />
+            <div className="mt-2 h-2.5 w-full rounded bg-neutral-800/40" />
+            <div className="mt-1.5 h-2.5 w-4/5 rounded bg-neutral-800/40" />
+          </div>
+        ))}
+      </div>
     </div>
   );
 }