Add GitHub Copilot instructions for repository-specific context

[Claude]

Provides coding guidelines and conventions to GitHub Copilot for better code generation aligned with this repository's patterns.

Added .github/copilot-instructions.md

Covers:

• Technology stack: Express/EJS/Node test runner stack with ESLint flat config
• Code patterns: Modern JS (destructuring, for...of, const by default), CommonJS modules, double quotes
• Security: Escaped EJS output (<%= %>), centralized error middleware
• Performance: Static asset caching, efficient iteration patterns (.entries() over C-style loops)
• Testing: Supertest with Node.js built-in test runner conventions

Example pattern documented:

// Prefer this
for (const [i, { image, text }] of haikus.entries()) {
  // Single destructure per iteration
}

// Over this
for (let i = 0; i < haikus.length; i++) {
  const image = haikus[i].image;  // Repeated lookups
}

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