- Add pytest configuration with coverage reporting
- Create test fixtures for mocking Vim environment
- Add unit tests for all Python modules:
  - test_utils.py: utility functions, logging, history
  - test_providers.py: all AI provider classes
  - test_tools.py: tool execution framework
  - test_core.py: main chat_gpt() function
  - test_summary.py: conversation summarization
- Add development dependencies (pytest, pytest-cov, pytest-mock)
- Extract summary.py module from VimScript to Python package
- Update core.py to use modular summary import

This completes the Python layer refactoring with full test coverage
for standalone testing without requiring Vim runtime.