Add PromptUnit SDK — drop-in OpenAI client with automatic model routing

[scorpion4ik26-lang]

Add PromptUnit SDK to API tools.

Drop-in OpenAI client for Node.js/TypeScript. Routes API calls to cheaper models automatically (GPT-4o-mini, Claude Haiku, Gemini Flash) with built-in fallback to OpenAI. 40-70% cost reduction, zero code changes.

import { createPromptUnit } from "@promptunit/sdk";
const client = createPromptUnit({ promptunitKey: "...", openaiKey: "..." });
// existing code works unchanged