refactor: remove gh/GitHub references from tokens, classes, and component names

Rename all --color-gh-* CSS tokens to --color-*, update Tailwind class names
(bg-gh-*, text-gh-*, border-gh-*, etc.) across all TSX/TS files, rename
GhButton component and file to Button, and remove trivial descriptive comments.

src/app/layout.tsx

@@ -13,7 +13,7 @@ export default function RootLayout({
 }>) {
 	return (
 		<html lang="en">
-			<body className="bg-gh-base text-gh-primary antialiased">{children}</body>
+			<body className="bg-base text-primary antialiased">{children}</body>
 		</html>
 	);
 }