Add Core Agent and Model Interfaces

[dewitt]

Summary

Introduces the foundational TypeScript interfaces for the modular agent architecture. These interfaces (Agent, Model, AgentEvent) establish stable contracts without altering any existing runtime behavior.

Details

This is Part 2 of 4 PRs toward #20995.

This PR adds pure interface definitions and associated verification tests. It allows the core engine to transition toward a plug-and-play architecture where components interact through these defined boundaries rather than relying on concrete implementations.

Related Issues

Part 2 of 4 PRs toward #20995

How to Validate

Review the TypeScript definitions in packages/core/src/interfaces/ and confirm that they represent the correct boundaries for our architectural shift.

Pre-Merge Checklist

• Updated relevant documentation and README (if needed)
• Added/updated tests (if needed)
• Noted breaking changes (if any)
• Validated on required platforms/methods:
  • MacOS
    • npm run
    • npx
    • Docker
    • Podman
    • Seatbelt
  • Windows
    • npm run
    • npx
    • Docker
  • Linux
    • npm run
    • npx
    • Docker