Add comprehensive linting setup and enhanced GitHub workflow

[paris3200]

Summary

This PR introduces comprehensive code quality improvements and CI enhancements:

• Linting Setup: Added luacheck configuration with project-specific rules and exclusions
• Code Quality: Fixed all existing linting warnings across the codebase while preserving functionality
• CI Enhancement: Updated GitHub Actions workflow to include parallel linting alongside testing
• Documentation: Enhanced contributing guidelines with development tools and workflow information
• Build System: Added Makefile with convenient targets for testing, linting, and maintenance

Key Changes

Code Quality & Linting

• Added .luacheckrc with comprehensive configuration for Neovim/Lua standards
• Fixed all luacheck warnings while maintaining backward compatibility
• Improved code readability and consistency throughout the codebase

CI/CD Improvements

• Renamed workflow from "Tests" to "CI" for broader scope
• Added parallel lint job that installs luacheck and validates code quality
• Updated test job to use Makefile commands for consistency
• Both jobs run on push/PR to main and develop branches

Developer Experience

• Added Makefile with targets: test, lint, lint-fix, clean, check-deps
• Updated README contributing section with modern development workflow
• Documented all available development tools and their usage

Test Plan

• All existing tests pass with new linting setup
• Linting passes on all source and test files
• GitHub workflow validates both testing and linting in CI
• Makefile targets work correctly for development workflow
• Documentation accurately reflects new development process

This PR establishes a solid foundation for code quality and makes the project more maintainable for future contributions.