Add rules-based formula for council_tax_benefit

The council_tax_benefit variable currently has no formula:

class council_tax_benefit(Variable):
    ...
    adds = ["council_tax_benefit_reported"]

This means it only returns a value for FRS-reported recipients — household-calculator users can't get a calculated CTR for hypothetical households, and any reform that changes income won't update CTR eligibility or amount.

Council Tax Reduction is means-tested. Each local authority sets its own scheme (since 2013), but pensioners are still on a national default scheme, and most working-age schemes track UC eligibility closely.

A first cut could:

• Use UC/Pension Credit eligibility as a proxy for full CTR eligibility (most LAs operate a reduction proportional to gross CT for working-age claimants and a 100% rebate for pensioners on the default scheme)
• Default to the national pensioner scheme rules
• Allow per-LA parameterisation as a follow-up

Found while reviewing PolicyEngine/policyengine-uk#1668.

Start by locating the council_tax_benefit Variable definition shown in the issue and inspect the existing UC and Pension Credit eligibility variables it could use. Determine the applicable default pensioner and working-age rules, then add tests covering hypothetical households and income changes; done means the formula produces a calculated CTR rather than only reported values.