feat: README architecture diagrams + React Flow diagram studio

Replace the tall portrait README ingestion SVG with two landscape
diagrams — "1 · Ingestion" (build the context layer) and "2 · Serving"
(agents query it through MCP) — wired in as transparent 2x PNGs that
read on GitHub light and dark.

Add docs-site/diagram-studio: a static React Flow page with custom
themed nodes and the inlined ktx mascot that renders both diagrams and
exports them to PNG via html-to-image (the diagrams' reproducible
source). Remove the superseded ingestion-flow SVGs.

docs-site/app/diagram-studio/page.tsx

@@ -0,0 +1,12 @@
+import type { Metadata } from "next";
+
+import { DiagramStudio } from "@/components/diagram-studio/studio";
+
+export const metadata: Metadata = {
+  title: "Diagram studio",
+  robots: { index: false, follow: false },
+};
+
+export default function DiagramStudioPage() {
+  return <DiagramStudio />;
+}