Update AI SDK to v5 and add repository guidelines

This PR updates the project to use AI SDK v5 and adds comprehensive repository guidelines. Key changes include:

1. Added AGENTS.md with detailed repository guidelines covering:

   • Project structure and organization
   • Build and development commands
   • Coding style conventions
   • Testing guidelines
   • Commit and PR best practices
   • Security considerations

2. Updated AI SDK dependencies:

   • Upgraded from v4 to v5 (ai@5.0.8)
   • Updated all provider packages to v2 series
   • Added explicit zod dependency with pinned version

3. Improved build script to:

   • Properly externalize dependencies
   • Add shebang line more reliably
   • Make output executable

4. Updated code to support AI SDK v5 API changes:

   • Migrated from ProviderV1 to ProviderV2 interfaces
   • Updated message conversion logic
   • Fixed stream handling for new event types
   • Made signature field optional in thinking content

5. Updated examples in documentation to reference newer models like gpt-5-mini instead of older models.

These changes ensure compatibility with the latest AI SDK while providing clear guidelines for contributors.