docs: clarify payroll and configuration guidance

[ryanduguid]

Summary

• Clarify that the implemented payroll tools use the New Zealand API only.
• Correct the Windows Claude Desktop JSON path example so every backslash is escaped.
• Tighten Custom Connection and bearer-token configuration guidance.

Validation

• npm run build
• npm run lint
• npm test

[ryanduguid]

Closing this one to cut the review load I created here.

I opened a large batch against this repository on 6 August. That was more than any maintainer could reasonably work through, and it was the wrong way to contribute. I have kept open the few changes with the clearest user impact and closed the rest, including this one.

The branch stays on my fork and the change still applies, so if this one is useful, say so and I will reopen it.