Set up GitHub Actions CI workflow with Elixir testing best practices

[Copilot]

This PR adds comprehensive GitHub Actions CI/CD workflows for the html2markdown project, following Elixir testing best practices for ecto-less projects.

Changes Made

🚀 Main CI Workflow (.github/workflows/ci.yml)

• Multi-version testing matrix: Tests across Elixir 1.15-1.17 and OTP 25-27 combinations
• Dependency caching: Optimized build times with proper Mix dependency and build caching
• Code quality enforcement:
  • Formatter checks (mix format --check-formatted)
  • Linting with Credo (mix credo --only warning)
  • Security analysis with Sobelow (mix sobelow --config)
• Comprehensive testing: Separate jobs for testing, coverage, and quality checks
• Coverage reporting: Generates HTML coverage reports and uploads as artifacts

⚡ Quick Test Workflow (.github/workflows/quick-test.yml)

• Manual trigger capability for development testing
• Weekly scheduled runs to catch dependency issues
• Focused on latest Elixir/OTP versions for fast feedback

🔧 Configuration Files

• .sobelow-conf: Security scanning configuration optimized for library projects
• README.md updates:
  • Added CI status badge
  • Added development section with testing and quality check commands
  • Documented the CI/CD setup

📊 Quality Assurance Features

• Matrix testing: Ensures compatibility across supported Elixir/OTP versions
• Security permissions: Follows GitHub security best practices with minimal required permissions
• Artifact management: Coverage reports are uploaded and retained for 30 days
• Integration with existing tooling: Uses the project's existing mix quality alias

Example Usage

# Run the same checks locally that CI runs
mix quality              # All quality checks
mix test                 # Run tests
mix coveralls.html       # Generate coverage report

The workflows will automatically run on:

• All pushes to main/master branches
• All pull requests
• Manual triggers (quick-test workflow)
• Weekly schedule (quick-test workflow)

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)
• repo.hex.pm
  • Triggering command: /usr/lib/erlang/erts-13.2.2.5/bin/inet_gethost 4 (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 my firewall allow list

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.