docs: Add comprehensive GitHub Copilot instructions for development workflow

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file that provides GitHub Copilot with detailed guidance on how to work effectively in the OpenFoodFacts Ruby SDK codebase.

What's included

The instructions cover everything a developer (or AI coding agent) needs to know when working with a fresh clone:

Environment Setup

• Ruby/Bundler installation with exact commands and PATH configuration
• User gem installation to avoid permission issues
• Bundle configuration for dependency management

Development Workflow

• Validated build and test commands with precise timing expectations:
  • bundle install: ~7-11 seconds
  • bundle exec rake test: ~2-3 seconds
  • bundle exec rake build: ~1 second
• All commands include "NEVER CANCEL" warnings with appropriate timeout buffers

Repository Knowledge

• Complete overview of the 29 model files in lib/openfoodfacts/
• Test infrastructure using VCR and webmock for HTTP request mocking
• GitHub workflows for CI (Ruby tests, RuboCop analysis)
• Key dependencies and version requirements

Validation Scenarios

• Step-by-step validation of core functionality (library loading, Product model usage)
• Manual testing approaches for network-restricted environments
• Troubleshooting guidance for common issues

Validation

Every single command and instruction has been thoroughly tested by following the exact setup sequence from a clean environment. The instructions successfully guide through the complete development workflow from initial setup to validating changes.

This ensures GitHub Copilot can work efficiently in this codebase with minimal trial and error, using only validated approaches rather than experimenting with uncertain commands.

Fixes #105.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• world.openfoodfacts.org
  • Triggering command: /usr/bin/ruby /tmp/test_openfoodfacts.rb (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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💡 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.

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