Add GitHub Copilot instructions for repository context

[Copilot]

Configures Copilot coding agent instructions per GitHub best practices to provide repository context and development guidelines.

Changes

• Added .github/copilot-instructions.md with:
  • Dual-language architecture (JS/TS and Python) structure
  • ESSE data convention and materials-only scope
  • Tools pattern: external parameters go in tools/, not class methods
  • Testing frameworks: Mocha (JS/TS), PyTest (Python)
  • Linting setup: ESLint + Prettier with pre-commit hooks
  • File format parsers: POSCAR, XYZ, Quantum ESPRESSO, CIF
  • Known platform-specific issues

Provides AI assistants and developers with essential context about the repository's domain (computational materials science), architecture patterns, and development workflows.

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 #260

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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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