feat: support cohere llm

[shukawam]

Issue: #30

Overview

• Added support for Cohere LLM.
• Using the OpenAI SDK because Cohere has an OpenAI-compatible endpoint.
  • Please let me know if using the Cohere SDK in the implementation is preferable.
• Maybe CI is failed because COHERE_API_KEY is not set to repo secret...

Usage example

import { agent, llmCohere } from "../dist/volcano-sdk.js";

// Run with: npx tsx examples/basic.ts
// (make sure COHERE_API_KEY is set)

(async () => {
  const llm = llmCohere({
    apiKey: process.env.COHERE_API_KEY!,
    model: "command-a-03-2025",
  });
  const results = await agent({ llm })
    .then({ prompt: `What is the Earch?` })
    .run();

  const firstResult = results[0];
  if (!firstResult || !firstResult.llmOutput) {
    throw new Error("The agent returned no results.");
  }
  console.log(firstResult.llmOutput);
})();