Create a pull request in darthjee/sinclair that adds GitHub Copilot instructions for the repository.

Goal

Add a Copilot instructions document (and/or configuration) so that Copilot-generated code, comments, and documentation follow the project’s conventions. The instructions must be written in English.

Requirements (must be included in the Copilot instructions)

1. Docker-based development workflow
   • Explain that development uses Docker and docker-compose.
   • Explain that developers use make dev to enter the dev environment (describe it succinctly, based on the repo’s tooling).
2. Project purpose
   • Explain the purpose of this gem: it is a foundation for developing other gems by providing base classes.
   • Read and reflect the repository README and the existing classes to ensure the description matches the code.
3. Language requirement
   • State clearly: all PRs, code, comments, and documentation must be written in English.
4. Tooling / CI
   • Mention that the project runs rspec, rubocop, and yardstick (confirm via .circleci/config.*).
   • Mention that documentation is generated with YARD.
5. Testing guidelines
   • Aim to have at least one spec for each file.
   • Note that files without specs are listed in config/check_specs.yml.
   • Prefer not to use mocks in specs.
   • Prefer one expectation per spec.
6. Code quality / style
   • Emphasize Clean Code practices.
   • Follow Sandi Metz Ruby principles: small methods, single responsibility classes, and avoid violating the Law of Demeter.

Implementation details

• Determine the correct location/format for Copilot instructions for this repository (e.g., .github/copilot-instructions.md or other GitHub-supported location).
• Add/update any necessary files so the instructions are discoverable by Copilot.
• Keep the instructions concise but actionable, using bullet points where appropriate.

Acceptance criteria

• A PR exists with the new Copilot instructions committed.
• Instructions are in English and cover all points above.
• Any references to commands/tools are accurate for this repo (validated by inspecting existing files like README.md, Makefile, docker-compose.yml, gem structure, and .circleci/config.yml).
• No unrelated formatting or whitespace churn.