Add .github/copilot-instructions.md for GitHub Copilot coding agent

[Copilot]

What does the PR do?

Adds .github/copilot-instructions.md — the standard file GitHub Copilot reads for repository-specific context. Covers project overview, directory structure, dev commands, env setup, code style, architecture patterns, testing, and CI/CD so Copilot can generate idiomatic code without per-prompt boilerplate.

Type of change

• Breaking Change
• Bug Fix
• New Feature

Linked Issue

Related Issue: #Set up Copilot instructions

Make sure you have

• Pulled from the default branch
• Documented your changes
• Linked the Issue
• Appointed a reviewer (if any)

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 #189

---

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