Add Oxford council tax reduction

[MaxGhenis]

Summary

• Add Oxford working-age Council Tax Reduction income-band parameters and variables.
• Register Oxford in the local CTR selector and simulated CTR aggregator.
• Add source-linked tests for income bands, Child Benefit disregard, tariff income, UC tariff exclusion, capital cutoff, and non-dependant deductions.

Sources

• Oxford Council Tax Reduction Scheme 2026 to 2027: https://www.oxford.gov.uk/council-tax-reduction/council-tax-reduction-scheme-2026-27
• Oxford 2026/27 appendices for UC treatment: https://www.oxford.gov.uk/council-tax-reduction/council-tax-reduction-scheme-2026-27/4
• Oxford public summary page: https://www.oxford.gov.uk/benefits-support/council-tax-reduction-scheme

Tests

• uvx ruff format --check .
• 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/oxford
• git diff --check
• 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