Add .github/copilot-instructions.md to configure Copilot coding agent

[Copilot]

Adds repository-specific instructions for the GitHub Copilot coding agent as recommended in the best practices guide.

Description

New file: .github/copilot-instructions.md covering:

• Project overview — electrostatic forward modeling library; core get_transformation_matrix() pattern returning M such that Y = M @ I
• Repository structure — annotated directory tree mapping source, tests, docs, examples, and workflows
• Architecture patterns — CellGeometry, LinearModel base class, subclassing contract, EEG/MEG dipole-moment pipeline
• Physical units table — nA (currents), µm (space), mV (potentials), S/m (conductivity), nA·µm (dipole moment), nA/µm (B-field)
• Coding conventions — PEP 8, 127-char line limit, NumPy docstrings, import ordering
• Dev commands — install, flake8 lint invocations matching CI, py.test -v test runner
• Dependencies — numpy, scipy, sympy, MEAutility
• Contributing pointers — CONTRIBUTING.md, PR template, pre-PR checklist

Related Issue

Closes # (set up Copilot instructions)

Motivation and Context

Without a copilot-instructions.md, Copilot has no project-specific context: it doesn't know the unit conventions, the mandatory get_transformation_matrix() contract, the flake8 line-length setting, or how to run tests. This file gives the agent enough domain knowledge to generate consistent, correct contributions.

How Has This Been Tested

Documentation-only change; no executable code modified.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

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)