Add GitHub Copilot instructions for repository

[Copilot]

Configures GitHub Copilot coding agent instructions per best practices documentation.

Changes

• Added .github/copilot-instructions.md with concise guidance for Copilot:
  • Project structure and monorepo layout
  • WordPress coding standards (PHP, JavaScript, HTML)
  • Key conventions: no namespaces, procedural patterns, @since n.e.x.t tags
  • Development commands for building, linting, testing
  • PHP 7.2+ type hints and PHPStan level 10 compliance
  • ES modules with JSDoc/TypeScript types
  • References to existing AGENTS.md and handbook

The file complements the existing AGENTS.md by providing Copilot-specific quick reference for common development tasks and coding standards.

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 #2351

---

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

[adamsilverstein]

I'm in favor of adding some agent guidelines, however maybe we should use the more generic AGENTS.md?

[westonruter]

@adamsilverstein I initiated this PR due to Copilot prompting me to do so in #2346:

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

I defer to your expertise for the best way to do this.

We do already have an AGENTS.md introduced in #2193, so maybe Copilot is just not aware of it?

[westonruter]

Ah, according to the PR description:

The file complements the existing AGENTS.md by providing Copilot-specific quick reference for common development tasks and coding standards.

[westonruter]

I'm not sure why having something Copilot-specific is better.