Add Montana child credit reform analysis notebooks

[DTrim99]

Summary

• Add analysis notebook for Montana newborn credit ($1,000/child under 1)
• Add analysis notebook for Montana young child credit ($1,000/child under 6)

Both notebooks analyze:

• Children eligible (under age threshold)
• Children benefiting from reform
• Budgetary impact
• Poverty impact (overall and child)
• Winners/losers distribution

Uses MT state dataset from HuggingFace.

Test plan

• Run newborn_credit_analysis.ipynb
• Run young_child_credit_analysis.ipynb
• Verify outputs are reasonable

Closes #121

🤖 Generated with Claude Code

[DTrim99]

Updates

Added new Montana Child Tax Credit analysis and enhanced all three MT notebooks:

New: ctc_analysis.ipynb

• Analyzes the MT CTC reform ($1,800 per child under 6, $1,500 per child 6-17)
• Comprehensive impact metrics matching KYPA format:
  • Budgetary impact with validation ratio
  • Person-level winners/losers (consistent with intra-decile)
  • Income decile analysis (average and relative)
  • Intra-decile distribution
  • Poverty impacts (overall, deep, child, deep child)
  • Income bracket breakdown

Enhanced: All three MT notebooks

Added Households and Children section showing:

• Total households with children
• Households with children benefiting (count and %)
• Total children (under 18)
• Children benefiting (count and %)

This addresses the need for child-specific metrics in the reform analysis.