add .github/copilot-instructions.md file

https://docs.github.com/en/copilot/customizing-copilot/adding-repository-custom-instructions-for-github-copilot

lib/fonts.ts

@@ -10,27 +10,14 @@ import {
 export const GeistSans = GeistSansLoader({
   subsets: ["latin"],
   display: "swap",
-  fallback: [
-    // https://github.com/system-fonts/modern-font-stacks#system-ui
-    "system-ui",
-    "sans-serif",
-  ],
+  variable: "--font-geist-sans",
   preload: true,
 });
 
 export const GeistMono = GeistMonoLoader({
   subsets: ["latin"],
   display: "swap",
-  fallback: [
-    // https://github.com/primer/css/blob/4113637b3bb60cad1e2dca82e70d92ad05694399/src/support/variables/typography.scss#L37
-    "ui-monospace",
-    "SFMono-Regular",
-    "'SF Mono'",
-    "Menlo",
-    "Consolas",
-    "'Liberation Mono'",
-    "monospace",
-  ],
+  variable: "--font-geist-mono",
   preload: true,
 });