Add an initial council tax reduction framework

[MaxGhenis]

Summary

• add an initial council_tax_reduction framework and wire it into council_tax_benefit
• support Wales, Scotland, English pension-age households, and explicit working-age overrides for Stroud and Dudley
• net Council Tax Reduction out of council_tax_less_benefit and hbai_household_net_income
• add coverage metadata, changelog, tests, and validation notes

Validation

• PYTHONPATH=/Users/maxghenis/worktrees/policyengine-uk-ctr policyengine-core test policyengine_uk/tests/policy/baseline/gov/local_authorities/council_tax_reduction/council_tax_reduction.yaml -c policyengine_uk
• PYTHONPATH=/Users/maxghenis/worktrees/policyengine-uk-ctr policyengine-core test policyengine_uk/tests/policy/baseline/household/consumption/council_tax_less_benefit.yaml -c policyengine_uk
• PYTHONPATH=/Users/maxghenis/worktrees/policyengine-uk-ctr pytest policyengine_uk/tests/code_health/test_variables.py -q

Entitledto spot checks

• Stroud spot check matched: entitledto returned full Council Tax Support on the billed liability and PolicyEngine returned the same annual reduction.
• Dudley did not match entitledto's current public calculator output. PolicyEngine follows Dudley Council's published 2025/26 scheme text and scheme PDF: 60% minimum payment, Band C cap for Band D+, and a flat GBP 5/week non-dependant deduction with the published disability exemption.

Notes

• This does not encode every working-age English CTR scheme yet.
• Unsupported English working-age authorities continue to fall back to reported baseline council_tax_benefit values instead of guessed rules.