Add Kingston upon Thames council tax reduction

[MaxGhenis]

Summary

• Add working-age Council Tax Reduction for Kingston upon Thames from the council's 2026-27 scheme PDF.
• Register Kingston as a supported local CTR scheme and include it in the local CTR aggregator.
• Reuse the shared legacy CTR helper, adding an optional additional applicable-income term for Kingston's non-UC capital tariff income.

Source

• Royal Borough of Kingston upon Thames, Council Tax Reduction Scheme 2026 - 2027: https://www.kingston.gov.uk/sites/default/files/2026-03/Council%20Tax%20Reduction%20Scheme%202026%20-%202027.pdf
• Modeled source facts: 100% maximum support, £16,000 capital limit, 20% withdrawal rate, £1/week tariff income per £250 or part above £6,000 for non-pensioners, 16-hour remunerative work gate, 2026 working-age non-dependant deduction schedule, UC maximum amount/income/capital treatment, and UC no-earned-income non-dependant exemption.

Tests

• uv run policyengine-core test policyengine_uk/tests/policy/baseline/gov/local_authorities/council_tax_reduction/council_tax_reduction.yaml -c policyengine_uk
• uv run ruff check policyengine_uk/variables/gov/local_authorities/council_tax_reduction policyengine_uk/variables/gov/local_authorities/kingston_upon_thames
• uv run policyengine-core test policyengine_uk/tests/policy -c policyengine_uk
• uv run --with pytest --with pytest-cov python -m pytest policyengine_uk/tests/ --cov=policyengine_uk --cov-report=xml --maxfail=1 -v
• git diff --check origin/main...HEAD