Add comprehensive tech-agnostic application description

[Copilot]

Adds APP_DESCRIPTION.md - a 1,568-word comprehensive description of the PoSnakeGame application covering all implemented features without referencing specific technologies or frameworks.

Content Coverage

• Game Mechanics: 48×32 arena, 30-second rounds, collision system, scoring
• Battle Royale: 15 AI opponents with 6 personality archetypes using Manhattan distance pathfinding with 30% randomness
• High Scores: Persistent leaderboard with offline-first architecture, automatic sync
• UI/UX: Responsive canvas rendering (8-20px cells), touch/keyboard controls, confetti celebrations
• Diagnostics: Health checks, masked configuration exposure, color-coded status indicators
• Offline: Client-side gameplay, 5s API timeouts with localStorage fallback, up to 20 cached scores
• Testing: Three-tier strategy (unit/integration/e2e) with container-based ephemeral databases
• Infrastructure: Static frontend + managed backend, NoSQL storage with date-range partitioning, credential-less auth via managed identities

Purpose

Provides a detailed, implementation-focused description suitable for documentation, presentations, or stakeholder communication without coupling to current tech stack choices.

Original prompt

Give me a 1500 word description of this app that includes every detail we discussed without specifying any particular tech stack

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.