Add 2026 tax config: New Zealand (nz)

[pascalwhoop]

Overview

New Zealand's FY2026-27 starts April 1, 2026. The current 2025 config covers FY2025-26. This issue tracks the FY2026-27 update.

Key FY2026-27 changes to research

• Income tax bracket thresholds and rates (confirm any changes)
• ACC earner levy rate and maximum earnings
• KiwiSaver contribution rates (employee/employer)

Acceptance criteria

• configs/nz/2026/base.yaml with accurate FY2026-27 parameters
• Test vectors pass npm run test:configs
• Sources cited from IRD (Inland Revenue) publications

Implementation

Use the /add-new-country skill. Start from the 2025 config. Target completion before April 1, 2026.