test: Add automated test suite with pytest and GitHub Actions CI workflow

[CodeMaverick-143]

Problem Description

The repository currently has zero unit tests and no automated CI workflow. Even though detector.py checks for PYTEST_CURRENT_TEST, there are no test files in the codebase to prevent regressions when PRs are submitted.

Proposed Solution

1. Add pytest and pytest-mock to dev dependencies.
2. Create tests/ directory with test cases for:
   • test_cli.py: Command line flags (--version, --help, non-existent path, output folder).
   • test_analyzer.py: AST parsing, comment counting (Python docstrings, C-style comments), function and class counting.
   • test_utils.py: list_source_files(), .insightignore filtering, supported file types.
   • test_reporter.py: Markdown file creation, metric formatting, summary generation.
   • test_detector.py: Mocked Gemini API responses and score parsing.
3. Setup .github/workflows/ci.yml running pytest across Python 3.9, 3.10, 3.11, 3.12, 3.13 on Ubuntu, macOS, and Windows.