Add copilot-instructions.md for workspace context

Provides Copilot with project overview, architecture, conventions, and a
development checklist so it has context from the start of the workshop.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: brunoborges

.github/copilot-instructions.md

@@ -0,0 +1,24 @@
+# Mona Mayhem — Copilot Workspace Instructions
+
+## Development Checklist (mandatory)
+1. `npm run build` — ensure project compiles
+2. `npm run dev` — verify dev server runs without errors
+
+## Project Overview
+- **What:** A retro arcade-themed GitHub Contribution Battle Arena
+- **Stack:** Astro v5, TypeScript, Node.js adapter (SSR)
+- **Entry point:** `src/pages/index.astro` — single-page battle UI
+- **API:** `src/pages/api/contributions/[username].ts` — proxies GitHub contribution data
+
+## Architecture
+- Astro SSR with `@astrojs/node` adapter in standalone mode
+- Single-page app with inline styles and interactive JavaScript
+- Server-side API route fetches GitHub contribution graphs with 1-hour caching
+- No external UI framework — plain HTML/CSS/JS in `.astro` files
+
+## Conventions
+- Use TypeScript for all new files
+- Keep styling scoped within Astro components or in dedicated CSS files
+- API routes go in `src/pages/api/`
+- Use semantic HTML elements for accessibility
+- Follow the retro arcade visual theme (pixel fonts, neon colors, scanline effects)