Add Cyber Monday trading window analyses and update documentation

Key Changes:
- Add COMPREHENSIVE_CYBER_MONDAY_ANALYSIS.md: 374 stocks, 8,510 observations, 10 statistically significant
- Add COMPARISON_CYBER_MONDAY_VS_THANKSGIVING.md: Side-by-side window comparison
- Add 3 Cyber Monday config files (DJIA, NASDAQ-100, S&P 500) with days_after=4
- Update README.md to document both trading windows (Thanksgiving vs Cyber Monday)
- Preserve all original Thanksgiving analyses on main branch

Analysis Details:
- Cyber Monday window extends to capture full e-commerce momentum (Monday after Thanksgiving)
- UNH shows strongest signal: p=0.001 (DJIA), 84% win rate, +2.80% median return
- 10 of 374 stocks reach statistical significance after Benjamini-Hochberg FDR correction
- Extended window shows higher returns but also higher volatility vs. traditional Black Friday exit

Technical Notes:
- Original Thanksgiving configs unchanged (days_after=1)
- Same statistical framework (Wilcoxon + BH FDR correction)
- All original analysis files preserved and untouched

Author: lieblm