OG image: embed actual book cover, add CTA, fix meta lengths

- Embed cover-l-1000x1400.png in OG image (was placeholder illustration)
- Add 'Read it free online' CTA button in OG image
- URL text: agents.siddhantkhare.com
- Meta title: 60 chars (was 46)
- Meta description: 136 chars (was 92)
- Remove SVG source (only PNG needed for OG)

Co-authored-by: Ona <no-reply@ona.com>

Author: Siddhant-K-code

public/cover.png

@@ -6,7 +6,7 @@ interface Props {
   description?: string;
 }
 
-const { title, description = "Concepts, workflows, and team practices for AI agent infrastructure. 10 Parts · 33 Chapters." } = Astro.props;
+const { title, description = "33 chapters on context engineering, agent security, cost control, and team practices for building AI agent infrastructure in production." } = Astro.props;
 const gaId = import.meta.env.PUBLIC_GA_ID || "G-XXXXXXX";
 const siteUrl = Astro.site?.toString().replace(/\/$/, "") || "";
 const base = import.meta.env.BASE_URL.replace(/\/$/, "");

public/og-image.png

@@ -19,7 +19,7 @@ const base = import.meta.env.BASE_URL.replace(/\/$/, "");
 const totalChapters = allChapters.filter((c) => c.chapter).length;
 ---
 
-<BaseLayout title="The Agentic Engineering Guide | Siddhant Khare">
+<BaseLayout title="The Agentic Engineering Guide | AI Agent Infrastructure Book">
   <Header />
 
   <main class="pt-14">