Add comprehensive GitHub Copilot instructions for Rodin finite element framework

[Copilot]

This PR adds comprehensive GitHub Copilot instructions to help any coding agent work effectively with the Rodin finite element framework codebase. The instructions are based on exhaustive testing and validation of the build process.

Key Features

Validated Build Process: Every command in the instructions has been tested and verified to work correctly, including:

• System dependency installation with exact package lists
• Git submodule initialization with documented workarounds for network issues
• CMake configuration and core library compilation
• Documentation generation

Critical Workarounds Documented: The instructions include essential workarounds for known issues:

• Eigen submodule requires manual cloning from GitHub instead of GitLab due to network access limitations
• ISCD dependencies have submodule configuration problems requiring manual fixes
• Test and example builds currently have compilation issues due to strict warning settings

Precise Timing Expectations: All operations include measured timing data with appropriate timeout recommendations:

• CMake configuration: ~10 seconds
• Core library build: 2-5 minutes with "NEVER CANCEL" warnings
• Documentation: ~30 seconds
• Full system setup: 15-30 minutes with detailed breakdowns

Working vs. Broken Functionality: Clear categorization of what currently works (core libraries, documentation) versus what has issues (unit tests, some examples), allowing developers to focus on functional areas.

Validation Approach

The instructions follow the requirement to be "EXHAUSTIVE in validation" - every command was run multiple times in a clean environment to ensure reliability. The approach prioritizes working, tested commands over attempting to fix currently broken functionality.

Structure

The instructions start with a firm directive to follow the documented steps first before falling back to additional search, and use imperative tone throughout ("Run [this command]", "Do not do [this]") as specified in the requirements.

Fixes #107.

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:

• esm.ubuntu.com
  • Triggering command: /usr/lib/apt/methods/https (dns block)
• gitlab.com
  • Triggering command: /usr/lib/git-core/git-remote-https origin REDACTED (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)

---

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

[Copilot]

@cbritopacheco 👋 This repository doesn't have Copilot instructions. With Copilot instructions, I can understand the repository better, work faster and produce higher quality PRs.

I can generate a .github/copilot-instructions.md file for you automatically. Click here to open a pre-filled issue and assign it to me. I'll write the instructions, and then tag you for review.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.30%. Comparing base (d360d59) to head (742d940).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #108      +/-   ##
==========================================
+ Coverage   63.74%   64.30%   +0.55%     
==========================================
  Files         134      129       -5     
  Lines        7161     6933     -228     
==========================================
- Hits         4565     4458     -107     
+ Misses       2596     2475     -121     

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