Add Copilot coding agent instructions

[Copilot]

Creates .github/copilot-instructions.md to provide repository context for the GitHub Copilot coding agent.

What's included

• Project overview — what Thundernetes does, key capabilities, beta status
• Repository structure — annotated directory tree covering all major components (cmd/, pkg/operator/, e2e/, etc.)
• Build & test commands — per-package unit test invocations matching CI, Docker build targets, local kind-based e2e setup
• Code conventions — frameworks in use (controller-runtime, Ginkgo/Gomega, gin, testify), operator patterns, CRD types, error handling and logging practices
• PR checklist — expected checks before submission (fmt/vet/tidy, install file regeneration, CLA)
• Go version constraint — note that Go 1.24 must stay consistent across go.mod, Dockerfiles, and workflow files

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 #495

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