chore: add readme for ts package

[joshsny]

No description provided.

[Copilot]

Pull Request Overview

This PR adds a comprehensive README.md file for the TypeScript package of the PostHog agent toolkit. The README provides installation instructions, quick start guides, and example usage patterns to help developers integrate PostHog with AI agents and LLM applications.

Key Changes

• Added complete documentation with installation and setup instructions
• Included integration examples for both Vercel AI SDK and LangChain.js frameworks
• Provided practical code examples for common use cases like analyzing user behavior and managing feature flags

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