Add Node.js quick-start example to README

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What needs to be done:
Add a Node.js code example to the README, similar to the existing Python example.

Where: README.md, after the Python Quick Start section.

Reference: The Node.js SDK is in sdk/node/. Look at the existing client class to understand the API.

Expected result:

const { AgentBillClient } = require('agentbill')
const client = new AgentBillClient({ apiKey: 'agb_your_key' })

const check = await client.preflight({ agentId: 'researcher', estimatedUnits: 10 })
if (!check.approved) throw new Error(`Blocked: ${check.reason}`)

// run your agent here

await client.record({ agentId: 'researcher', units: 10 })