Add Copilot instructions for repository

[Copilot]

Configures GitHub Copilot coding agent instructions per best practices documentation.

Changes

• Added .github/copilot-instructions.md documenting:
  • Project architecture (zero-dependency serverless framework for AWS Lambda)
  • Code conventions (ES6+, strict mode, single quotes, ESLint + Prettier)
  • Testing strategy (Jest unit tests, TypeScript type tests via tsd)
  • Build commands (npm test, npm run lint:check, npm run prettier:check)
  • AWS integration patterns (API Gateway v1/v2, ALB support)
  • Key constraints (no external dependencies, serverless-first design, backwards compatibility)

Emphasizes the non-negotiable zero-dependency philosophy and Express.js-like API patterns to guide code suggestions.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>✨ Set up Copilot instructions</issue_title>
<issue_description>Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ✨ Set up Copilot instructions #293

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