Add Copilot coding agent instructions

[Copilot]

GitHub Copilot coding agent had no repository context, leading to generic suggestions that may not align with this project's conventions.

Adds .github/copilot-instructions.md covering:

• Project overview — mod ID, root package (fr.hugman.mubble), Fabric + Gradle stack
• Module structure — mubble-core, mubble-super_mario, mubble-bom and the split source set layout (main / client / datagen)
• Coding conventions — Java 25 target, registry pattern, mixin placement, client/server source set separation
• Key dependencies — Fabric API, Dawn API, logging conventions
• Build commands — ./gradlew build, runDatagen, check
• Contributing rules — CLA, PR requirements, Crowdin-managed translations

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 #108

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