Fix abolishing council tax having no budget impact (#1153)

[MaxGhenis]

Summary

• Abolishing council tax now correctly shows budget impact
• Created council_tax_applicable variable that returns 0 when abolish_council_tax is True
• Replaced council_tax with council_tax_applicable in all downstream aggregation variables
• Removed special-case abolish_council_tax formula overrides (now unnecessary)

Root cause

council_tax was a pure input variable with no formula. The abolish_council_tax parameter removed it from aggregation sums in both household_tax AND pre_budget_change_household_tax, so the reform-vs-baseline delta was always zero.

Fix

New council_tax_applicable variable: returns 0 if abolished, otherwise returns the council_tax input. All downstream variables (gov_tax, household_tax, pre_budget_change_household_tax, hbai_household_net_income, household_benefits) now reference council_tax_applicable instead.

Test plan

• New test: abolishing council tax increases household_net_income
• Test fails on old code, passes on fix
• All existing tests pass

Fixes #1153