Add Copilot coding agent instructions

[Copilot]

Adds .github/copilot-instructions.md to give GitHub Copilot structured context about this repository so it can assist more accurately.

What's included

• Repo layout – maps src/, easysba/, tests/, and config files to their roles
• Build requirements – Python 3.11+, C toolchain, BLAS/LAPACK, install commands
• Test instructions – pytest invocation and the EASYSBA_RUN_SOLVER=1 flag required to run the solver tests
• Conventions – PEP 8 for Python, match existing style for C/C++, unittest.TestCase + pytest discovery
• Key files table – quick-reference mapping of the most important source files to their purpose

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 #9

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[Copilot]

Pull request overview

This pull request adds .github/copilot-instructions.md to provide structured context about the easySBA Python bindings repository for GitHub Copilot. The instructions document repository layout, build requirements, testing procedures, coding conventions, and key files to help Copilot assist more accurately with code generation and review tasks.

Changes:

• Added comprehensive Copilot instructions file documenting repository structure, build/test procedures, and coding conventions
• Included a key files reference table mapping important source files to their purposes
• Documented the EASYSBA_RUN_SOLVER environment variable requirement for solver tests

[Copilot]

The statement "Tests live in tests/ and use unittest.TestCase classes" is not entirely accurate. While test_smoke.py uses unittest.TestCase, test_sba_steps.py uses plain pytest functions with pytest decorators like @pytest.mark.parametrize. Consider updating this to reflect both testing styles: "Tests live in tests/ and use either unittest.TestCase classes or plain pytest functions, discovered and run via pytest."

Suggested change

	- Tests live in `tests/` and use `unittest.TestCase` classes, discovered and run via `pytest`.
	- Tests live in `tests/` and use either `unittest.TestCase` classes or plain pytest functions, discovered and run via `pytest`.