docs(cookbook): add in-situ 'Try it live' iframe to Daytona recipe

Embed the live demo directly in the cookbook recipe, mirroring the in-doc
iframe approach used by integration landing pages (framework-overview.tsx
liveDemos[] / IframeSwitcher). The recipe now opens with a 'Try it live'
section above the prerequisites, iframing the showcase deployment so
readers can drive the runCode tool against a real Daytona sandbox without
leaving the page.

URL is a placeholder ('showcase-daytona-runcode-production.up.railway.app',
matching the showcase-{slug}-production.up.railway.app backend host
pattern from the registry generator). Will render Railway's not-found
page until a permanent demo deployment is provisioned at that name; the
inline MDX comment notes the placeholder.

OSS-222

Author: contextablemark

showcase/shell-docs/src/content/docs/cookbook/daytona.mdx

@@ -13,6 +13,16 @@ the chat, so the agent can execute code it writes without touching your host.
 This guide assumes you already have a running Built-in Agent app. If you don't, follow the
 [Quickstart](/built-in-agent/quickstart) first — it takes a few minutes.
 
+## Try it live
+
+{/* Placeholder URL — update to the permanent demo deployment once it exists. */}
+
+<iframe
+  src="https://showcase-daytona-runcode-production.up.railway.app"
+  title="Daytona runCode live demo"
+  className="w-full h-[480px] sm:h-[600px] block rounded-xl border border-[var(--border)]"
+/>
+
 ## Prerequisites
 
 - A running Built-in Agent app (see the [Quickstart](/built-in-agent/quickstart)).