Remove macOS and Windows CI builds, keep only Ubuntu

[Copilot]

This PR simplifies the CI configuration by removing macOS and Windows build checks as requested, keeping only Ubuntu builds with GCC and Clang compilers.

Changes Made

• Simplified build matrix: Removed windows-latest and macos-latest from the OS matrix, keeping only ubuntu-latest
• Removed MSVC compiler: No longer included since Windows builds are removed
• Consolidated environment setup: Single Ubuntu setup step instead of OS-specific conditionals
• Removed Windows-specific dependencies: Eliminated vcpkg and Windows Boost installation steps
• Simplified compilation and test execution: Removed OS-specific logic and conditional file extensions
• Updated documentation: Modified .github/CI-README.md to reflect the Ubuntu-only setup

Impact

The CI workflow is now significantly simpler while maintaining comprehensive compiler coverage:

• Before: 3 operating systems × 2-3 compilers = 7 build jobs
• After: 1 operating system × 2 compilers = 2 build jobs

This reduces CI resource usage and complexity while still validating the header-only C++ library with both major compiler families (GCC and Clang). The existing code quality checks (format validation and header independence) remain unchanged and continue to run on Ubuntu.

Fixes #8.

Warning

Firewall rules blocked me from connecting to one or more addresses

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)
  • Triggering command: /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)

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