Enhance Copilot instructions with architecture, style, and development context

[Copilot]

Extended .github/copilot-instructions.md with critical context missing from the original 171-line file. Added 154 lines covering gaps that would cause Copilot to make incorrect assumptions about codebase structure and conventions.

Additions

• Code Style & Quality: Ruff configuration, legacy code handling, minimal-change philosophy
• Architecture: Dual-interface design (wxPython GUI vs scriptable API), Fortran extension compilation model
• CI/CD Context: Pixi-based workflows, test categorization (offline/network), platform matrix
• Contributing Patterns: Pre-change checklist, when rebuilds are required, PATH management for executables
• Common Patterns: Space group operations, lattice parameter handling, scriptable interface idioms
• Gotchas: Build timing expectations (12s-30min), executable PATH requirements, platform-specific compiler needs

Key Context for Copilot

The dual architecture is critical - files with *GUI.py suffix require display/X11 and wxPython, while core modules work headless. Fortran extensions in sources/ require full rebuild via meson/f2py. Tests need LATTIC and convcell executables in PATH.

File structure now: 324 lines, 12 major sections, 32 code examples.

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 #298

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[briantoby]

looks good to me