Add CI workflows for PR test coverage enforcement and Nix builds

[Copilot]

Implements automated enforcement of test coverage requirements (100% functions, 90% lines) on PRs to main, and automatic Nix builds on commits to main.

Changes

• .github/workflows/pr-validation.yml - Blocks PRs failing tests or not meeting coverage thresholds:

  • Runs full test suite via cmake --build build --target tests
  • Generates coverage report with llvm-cov
  • Fails workflow if coverage < 100% functions or < 90% lines

• .github/workflows/build-main.yml - Builds project on main branch:

  • Uses nix build for reproducible builds
  • Uploads engine binary as artifact (30d retention)

Both workflows use Nix development environment and include explicit contents: read permissions per security best practices.

Original prompt

Met en place :

• Blocage des pull requests si elles ne respectent pas les conventions de test du code définies dans @README.md
• Build automatique du projet avec Nix pour les commits sur main

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