Add Montana child credit reform analysis notebooks

[DTrim99]

Summary

Add analysis notebooks for Montana's proposed child credit reforms:

1. Newborn Credit (newborn_credit_analysis.ipynb)

   • $1,000 credit per child under age 1
   • Phase-out: $60k (single) / $120k (joint) AGI
   • Analysis for 2027

2. Young Child Credit (young_child_credit_analysis.ipynb)

   • $1,000 credit per child under age 6
   • Phase-out: $40k (single) / $80k (joint) AGI
   • Analysis for 2027

Metrics Analyzed

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

Dataset

Uses Montana state-level dataset from HuggingFace.