Add AgentsKit to Generative AI

[EmersonBraun]

What this adds

Adds AgentsKit to the Generative AI section as a modular TypeScript toolkit for building provider-independent AI agents.

Validation

• confirmed the repository is public, MIT-licensed, active, tested, and documented;
• searched the current list plus open and closed issues and pull requests for an existing AgentsKit entry;
• used the documented one-line package format and placed the entry at the bottom of the matching section;
• limited the change to one line in README.md.

I maintain AgentsKit and will respond to review feedback.