Add pytest test infrastructure and initial test coverage

[davidpoblador]

Summary

No tests currently exist in the project. This is a critical gap that prevents safe refactoring and regression detection.

Tasks

• Add pytest and pytest-cov to dev dependencies in pyproject.toml
• Create tests/ directory structure
• Create conftest.py with shared fixtures
• Add initial test coverage for core modules:
  • core/post.py - BlogPost model parsing
  • core/blog.py - BlogConfig loading and post discovery
  • core/site.py - Feed generation
• Configure pytest in pyproject.toml
• Target 80%+ coverage for core functionality

Priority

Critical

Files to create

• tests/__init__.py
• tests/conftest.py
• tests/test_post.py
• tests/test_blog.py
• tests/test_site.py