feat(extractor): Add Excel conversion and post-processing capabilities

Add comprehensive post-processing features to convert downloaded Excel files
into structured data and merge them into a single output file.

New modules:
- extractor_core.py: Stateless pure functions for web operations
- excel_converter.py: Excel to DataFrame conversion utility
- tests/test_extractor_real.py: Real data extraction test suite

Enhanced features:
- post_process_downloads(): Convert and merge multiple Excel files
- extract_and_process(): Complete workflow in single call
- cleanup_temp_files(): Optional cleanup of temporary downloaded files
- Field name mapping for standardized output columns

Dependencies:
- pandas>=2.0.0 for data manipulation
- openpyxl>=3.1.0 for Excel file handling

Documentation:
- Updated CLAUDE.md with new module references
- Added API documentation for extractor components

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

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