Add GitHub Copilot instructions

[Copilot]

Adds .github/copilot-instructions.md to provide codebase context for GitHub Copilot.

Contents

• Tech stack: C++17, GTK+3, Meson build system, Google Test
• Code standards: Chromium-style clang-format, warning configurations from meson.build, EditorConfig settings
• Build & test: Meson commands, sanitizer builds, modular test structure
• Architecture: Core components (CoreThread, IptuxConfig, ProgramData, TransFileModel, Event)
• Platform specifics: Linux CI matrix (Ubuntu 22.04/24.04/latest, g++/clang++), macOS with Homebrew
• Development workflows: Feature addition, bug fixes, UI changes with translation considerations

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 #700

---

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

[github-actions]

Test Results

69 tests  ±0   69 ✅ ±0   3s ⏱️ ±0s
32 suites ±0    0 💤 ±0 
 1 files   ±0    0 ❌ ±0 

Results for commit 353e3cf. ± Comparison against base commit 89e5243.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.00%. Comparing base (89e5243) to head (353e3cf).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #701   +/-   ##
=======================================
  Coverage   52.00%   52.00%           
=======================================
  Files          64       64           
  Lines        8599     8599           
=======================================
  Hits         4472     4472           
  Misses       4127     4127           

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