Refactor reforms to use two-layer pattern with bypass parameter

[MaxGhenis]

Summary

Adopt the PolicyEngine US reform pattern and remove all gov.contrib references from gov/ variables. This provides a clean separation between core law (gov/) and policy proposals (contrib/).

Pattern Changes

The new pattern uses:

1. A create_*() function that returns the Reform class directly
2. A create_*_reform() wrapper that checks parameters and calls the creator
3. A bypass parameter that allows skipping parameter checks for programmatic use

New Reform Files Created

Reform	Purpose
policyengine/abolish_council_tax.py	Removes council_tax from gov_tax/household_tax
policyengine/freeze_pension_credit.py	Returns baseline pension credit values
policyengine/employer_ni_pension_exemption.py	Exempts employer pension contributions from NI
policyengine/salary_sacrifice_haircut.py	Applies broad-base employment income haircut
policyengine/two_child_limit_age_exemption.py	Handles CTC/UC child limit age exemptions
ubi_center/basic_income_interactions.py	Handles UBI means-testing, taxation, and CB withdrawal

Base Variables Updated

The following variables had gov.contrib conditional logic removed:

• gov_tax.py, household_tax.py (abolish_council_tax)
• child_benefit_respective_amount.py (withdraw_cb)
• adjusted_net_income.py (include_in_taxable_income)
• tax_credits_applicable_income.py, pension_credit_income.py, income_support_applicable_income.py, housing_benefit_applicable_income.py, uc_mif_capped_earned_income.py (include_in_means_tests)
• pension_credit.py (freeze_pension_credit)
• uc_individual_child_element.py, is_CTC_child_limit_exempt.py (two_child_limit age exemption)
• ni_class_1_employer.py (employer_ni pension exemption)
• salary_sacrifice_broad_base_haircut.py (haircut rate)

Example Usage

# Direct import (no parameter checks)
from policyengine_uk.reforms import abolish_council_tax_reform

# Create with bypass
from policyengine_uk.reforms import create_abolish_council_tax_reform
reform = create_abolish_council_tax_reform(parameters, period, bypass=True)

# Standard parameter-driven creation
reform = create_abolish_council_tax_reform(parameters, period)  # Only returns if contrib param is set

Test Plan

• All 51 tests pass (excluding pre-existing UC taper rate failure)
• All reform imports work correctly
• bypass=True returns reforms without parameter checks
• Pre-instantiated reforms available for direct import
• No gov.contrib references remain in gov/ variables

🤖 Generated with Claude Code