docs: add generated og:image for the site (#163)

## Summary

Adds a social/OG card for the site, generated deterministically with
`next/og` (no AI image gen, so the logo and text are exact and crisp).

`site/app/opengraph-image.tsx` renders a 1200×630 PNG with:
- the **Codegen brand mark**,
- the **Graph-sitter** wordmark,
- the tagline **"Industrial-grade static analysis toolkit"**, and
- a **node-graph network** background with a purple glow.

Next automatically wires this file into `og:image` and `twitter:image`
(the layout already declares `metadataBase` + a `summary_large_image`
twitter card), so it covers `/`, `/docs`, and `/explore`.

## Test plan
- [x] `npm run build` in `site/` passes; `/opengraph-image` prerenders
to a valid 1200×630 PNG.
- [ ] Verify the rendered card visually (locally hit
`/opengraph-image`).
- [ ] Verify the card in a social/OG debugger after deploy.

---------

Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: jayhack <2548876+jayhack@users.noreply.github.com>

Author: jayhack

site/app/opengraph-image.tsx

@@ -0,0 +1,140 @@
+import { ImageResponse } from "next/og";
+
+export const alt = "Graph-sitter — industrial-grade static analysis toolkit";
+export const size = { width: 1200, height: 630 };
+export const contentType = "image/png";
+
+const WIDTH = 1200;
+const HEIGHT = 630;
+
+// Deterministic node-graph laid out mostly to the right so it reads as a
+// network behind the wordmark without fighting the text on the left.
+const nodes: { x: number; y: number; r: number; c: string; o: number }[] = [
+	{ x: 120, y: 92, r: 5, c: "#a277ff", o: 0.5 },
+	{ x: 78, y: 520, r: 6, c: "#82e2ff", o: 0.45 },
+	{ x: 300, y: 560, r: 5, c: "#61ffca", o: 0.5 },
+	{ x: 760, y: 92, r: 7, c: "#a277ff", o: 0.95 },
+	{ x: 900, y: 168, r: 9, c: "#82e2ff", o: 0.95 },
+	{ x: 1044, y: 110, r: 6, c: "#ffca85", o: 0.9 },
+	{ x: 980, y: 300, r: 12, c: "#a277ff", o: 0.95 },
+	{ x: 1124, y: 380, r: 7, c: "#61ffca", o: 0.9 },
+	{ x: 860, y: 432, r: 8, c: "#82e2ff", o: 0.95 },
+	{ x: 1020, y: 520, r: 6, c: "#a277ff", o: 0.9 },
+	{ x: 724, y: 560, r: 7, c: "#ffca85", o: 0.9 },
+	{ x: 700, y: 300, r: 9, c: "#a277ff", o: 0.95 },
+	{ x: 560, y: 198, r: 5, c: "#6f6b80", o: 0.6 },
+	{ x: 600, y: 460, r: 6, c: "#82e2ff", o: 0.85 },
+	{ x: 1140, y: 232, r: 5, c: "#6f6b80", o: 0.6 },
+	{ x: 840, y: 250, r: 5, c: "#61ffca", o: 0.85 },
+	{ x: 470, y: 92, r: 4, c: "#6f6b80", o: 0.5 },
+	{ x: 432, y: 540, r: 5, c: "#a277ff", o: 0.7 },
+];
+
+const edges: [number, number][] = [
+	[3, 4],
+	[4, 5],
+	[4, 6],
+	[6, 7],
+	[6, 8],
+	[8, 9],
+	[8, 10],
+	[6, 11],
+	[11, 3],
+	[11, 15],
+	[15, 4],
+	[8, 13],
+	[13, 10],
+	[11, 12],
+	[12, 16],
+	[11, 5],
+	[6, 14],
+	[7, 9],
+	[10, 9],
+	[8, 15],
+	[13, 17],
+	[17, 2],
+	[12, 0],
+	[1, 17],
+	[14, 7],
+	[3, 15],
+];
+
+function networkDataUri() {
+	const lines = edges
+		.map(([a, b]) => {
+			const p = nodes[a];
+			const q = nodes[b];
+			return `<line x1="${p.x}" y1="${p.y}" x2="${q.x}" y2="${q.y}" stroke="#8f74ff" stroke-width="1.4" stroke-opacity="0.42"/>`;
+		})
+		.join("");
+	const dots = nodes
+		.map(
+			(n) =>
+				`<circle cx="${n.x}" cy="${n.y}" r="${n.r}" fill="${n.c}" fill-opacity="${n.o}"/>`,
+		)
+		.join("");
+	const svg = `<svg xmlns="http://www.w3.org/2000/svg" width="${WIDTH}" height="${HEIGHT}" viewBox="0 0 ${WIDTH} ${HEIGHT}"><defs><radialGradient id="glow" cx="32%" cy="42%" r="68%"><stop offset="0%" stop-color="#7c4dff" stop-opacity="0.42"/><stop offset="48%" stop-color="#7c4dff" stop-opacity="0.10"/><stop offset="100%" stop-color="#0a0a0f" stop-opacity="0"/></radialGradient></defs><rect width="${WIDTH}" height="${HEIGHT}" fill="#0a0a0f"/><rect width="${WIDTH}" height="${HEIGHT}" fill="url(#glow)"/><g>${lines}</g><g>${dots}</g></svg>`;
+	return `data:image/svg+xml,${encodeURIComponent(svg)}`;
+}
+
+const markSvg = `<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 80.87 80.87" fill="#ffffff"><path d="M57.34,27.51c-.79.79-.79,2.07,0,2.86l8.64,8.64c.79.79.79,2.07,0,2.86l-8.64,8.64c-.79.79-.79,2.07,0,2.86l4.29,4.29c.79.79,2.07.79,2.86,0l15.79-15.79c.79-.79.79-2.07,0-2.86l-15.79-15.79c-.79-.79-2.07-.79-2.86,0,0,0-4.29,4.29-4.29,4.29Z"/><path d="M50.19,60.51c-.79-.79-2.07-.79-2.86,0l-5.47,5.47c-.79.79-2.07.79-2.86,0l-24.12-24.12c-.79-.79-.79-2.07,0-2.86l24.12-24.12c.79-.79,2.07-.79,2.86,0l5.47,5.47c.79.79,2.07.79,2.86,0l4.29-4.29c.79-.79.79-2.07,0-2.86L41.86.59c-.79-.79-2.07-.79-2.86,0L.59,39.01c-.79.79-.79,2.07,0,2.86l38.41,38.41c.79.79,2.07.79,2.86,0l12.62-12.62c.79-.79.79-2.07,0-2.86,0,0-4.29-4.29-4.29-4.29Z"/><path d="M50.54,40.44c0,5.58-4.53,10.11-10.11,10.11s-10.11-4.53-10.11-10.11,4.53-10.11,10.11-10.11,10.11,4.53,10.11,10.11Z"/></svg>`;
+const markUri = `data:image/svg+xml,${encodeURIComponent(markSvg)}`;
+
+export default function OpengraphImage() {
+	return new ImageResponse(
+		<div
+			style={{
+				width: "100%",
+				height: "100%",
+				display: "flex",
+				position: "relative",
+				backgroundColor: "#0a0a0f",
+			}}
+		>
+			{/* biome-ignore lint/a11y/useAltText: decorative background in OG image */}
+			<img
+				src={networkDataUri()}
+				width={WIDTH}
+				height={HEIGHT}
+				style={{ position: "absolute", top: 0, left: 0 }}
+			/>
+
+			<div
+				style={{
+					display: "flex",
+					flexDirection: "column",
+					justifyContent: "center",
+					height: "100%",
+					padding: "0 96px",
+				}}
+			>
+				<div style={{ display: "flex", alignItems: "center" }}>
+					{/* biome-ignore lint/a11y/useAltText: decorative logo in OG image */}
+					<img src={markUri} width={104} height={104} />
+					<div
+						style={{
+							marginLeft: 36,
+							fontSize: 98,
+							fontWeight: 700,
+							color: "#ffffff",
+							letterSpacing: -3,
+						}}
+					>
+						Graph-sitter
+					</div>
+				</div>
+				<div
+					style={{
+						marginTop: 26,
+						fontSize: 42,
+						color: "#c4c1cc",
+						letterSpacing: -0.5,
+					}}
+				>
+					Industrial-grade static analysis toolkit
+				</div>
+			</div>
+		</div>,
+		{ ...size },
+	);
+}