docs(nitrosend): align plan and prepaid billing guidance

[auscaster]

Summary:

• align the public billing-status guidance with the live separate funding block
• publish Nitrosend skill version 0.4.6 and regenerate official locks
• make prepaid funding and add-funds preservation a load-bearing fixture assertion

Architecture:

• plan purchasing remains the first-class Runx lifecycle
• prepaid funding remains a separate Nitrosend MCP path
• no new client, adapter, runner, or settlement layer

Verification:

• complete 20-case Nitrosend harness
• pnpm verify:fast (251 tests)
• mutation proof: renaming the mocked funding key fails at the new nested assertion
• two passing independent scafld reviews