Add Copilot coding agent instructions

[Copilot]

Adds .github/copilot-instructions.md to give the Copilot coding agent repository-specific context, reducing the need for it to infer conventions from scratch on each task.

What's included

• Build: How to invoke build.ps1 / build.sh and what flags are available (-SkipTests, -SkipPack, -Configuration)
• Testing: Per-project dotnet test commands for all three test projects
• Code conventions: Async naming, logging infrastructure, HTTP access patterns (HttpSource), CancellationToken usage
• Project structure: Annotated directory tree covering src/, test/, and build/
• Key APIs: Public surface of CatalogReader and FeedReader
• CI/CD: Summary of dotnet.yml (build/test on push/PR) and publish.yml (manual NuGet.org publish)

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 #40

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