Enhance Copilot instructions with project context and best practices

[Copilot]

The repository lacked comprehensive Copilot instructions to guide code generation and reviews effectively. Enhanced .github/copilot-instructions.md to follow GitHub's best practices for custom instructions.

Added Context Sections

• Project Overview: Describes Gladius as a volumetric 3MF playground with CSG interface, rendering engine, and multi-language API bindings
• Tech Stack: Documents C++11, CMake, vcpkg, OpenGL/OpenCL dependencies with categorized library list
• Project Structure: Maps directory layout (src/, library/, tests/, etc.)
• Build Instructions: Platform-specific commands for Windows/Linux with prerequisites and environment variables

Added Code Examples

Concrete C++ patterns for project conventions:

// Naming: PascalCase classes, lowerCamelCase functions, m_ private members
class MeshRenderer {
private:
    int m_vertexCount;  // m_ prefix for private
public:
    void renderMesh();  // lowerCamelCase, verb prefix
    bool isVisible() const;  // is prefix for booleans
};

// East-side const preference
int const* ptr;  // Preferred over const int*

// Smart pointers over raw pointers
std::unique_ptr<Mesh> createMesh() {
    return std::make_unique<Mesh>();
}

Preserved Existing Guidelines

All original C++ guidelines remain: Allman braces, 4-space indents, GTest conventions, Doxygen comments, and test naming patterns (UnitOfWork_StateUnderTest_ExpectedBehavior).

File size: 223 lines (well under 1000 line recommendation).

Original prompt

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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 #16

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