Design unified provider interface (base classes, protocols)

[Aureliolo]

Context

Design the abstract interface that all LLM providers must implement. This is the foundation of the provider layer — every provider adapter (Anthropic, OpenRouter, Ollama, etc.) will conform to this interface, ensuring consistent behavior across the framework.

Acceptance Criteria

• Abstract base class or Protocol defining the unified provider interface
• completion(messages, tools, config) -> response signature per spec 9.1
• Streaming support in the interface design
• Provider configuration model (API keys, base URLs, timeouts, etc.)
• Model capability description (context window size, tool use support, vision support)
• Cost information interface (input/output token pricing per model)
• Unit tests for interface contracts and config validation

Dependencies

None — this is the foundation for all provider work.

Design Spec Reference

Section 9.1 — Provider Interface Design