Initial commit: Snake game implementation

Co-Authored-By: Warp <agent@warp.dev>

AGENTS.md

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+# AGENTS.md
+
+This file provides guidance to WARP (warp.dev) when working with code in this repository.
+
+## Project Overview
+
+This is a Snake game implemented with vanilla JavaScript, HTML5 Canvas, and CSS. The project has no build system or dependencies - it runs directly in the browser.
+
+## Running the Game
+
+Open `index.html` directly in a web browser:
+```bash
+open index.html  # macOS
+```
+
+Or use a simple HTTP server for development:
+```bash
+python3 -m http.server 8000
+# Then visit http://localhost:8000
+```
+
+## Architecture
+
+### Game State Management
+The game uses an immutable state pattern managed in `script.js`:
+- **State object**: Contains `snake` (array of segments), `dir` (current direction), `nextDir` (queued direction), `food` position, `score`, and `over` flag
+- **Pure functions**: `step()` takes current state and returns new state without mutations
+- **Game loop**: `setInterval` calls `step()` every 120ms, then `render()` updates canvas and DOM
+
+### Key Game Logic
+- **Direction handling**: Prevents 180-degree turns via `isOpposite()` check
+- **Collision detection**: `outOfBounds()` and `hitSelf()` determine game over conditions
+- **Food placement**: `placeFood()` finds random empty cells by excluding snake positions
+- **Growth mechanism**: When snake eats food, tail segment isn't removed in next step
+
+### Rendering Pipeline
+1. `drawGrid()` - Clears and draws background grid
+2. `drawFood()` - Renders food as a red circle
+3. `drawSnake()` - Draws snake segments (darker green for head)
+4. UI updates via direct DOM manipulation (`scoreEl`, `statusEl`)
+
+### Input Handling
+- Arrow keys and WASD mapped in `directions` object
+- `nextDir` queues direction changes to prevent mid-tick direction updates
+- Enter key restarts game when over
+
+## Code Conventions
+
+- Uses IIFE to avoid global scope pollution
+- Functional approach for game logic (pure `step()` function)
+- Grid coordinates: 20x20 grid, each cell is 20px
+- Color scheme defined in CSS custom properties in `styles.css`