Add Copilot instructions for repository context

[Copilot]

Adds .github/copilot-instructions.md to provide Copilot with repository-specific context about LightGBM's architecture, conventions, and development workflows.

Coverage

• Structure: Core components (src/, include/, python-package/, R-package/, tests/)
• Languages: C++17 standards, Python 3.9+ conventions, R package patterns
• Build: CMake options, compilation flags, build commands
• Testing: Google Test (C++), pytest (Python) patterns and fixtures
• Linting: cpplint, ruff, mypy, pre-commit hooks configuration
• Architecture: Key classes (Boosting, Dataset, TreeLearner, etc.) and their roles
• Patterns: Dataset creation, model training, C API usage examples
• Anti-patterns: Common mistakes to avoid (e.g., modifying external_libs/, committing generated files)

File Location

.github/copilot-instructions.md  (351 lines)

Structured as reference documentation that Copilot can use to generate context-appropriate suggestions aligned with existing project conventions.

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:

• gh.io
  • Triggering command: /home/REDACTED/work/_temp/ghcca-node/node/bin/node /home/REDACTED/work/_temp/ghcca-node/node/bin/node --enable-source-maps /home/REDACTED/work/_temp/copilot-developer-action-main/dist/index.js (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)

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 #7172

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[jameslamb]

pre-emptively leaving a blocking review so I have a chance to review again if this is ever taken out of draft.