Add agent interface and model implementations

Implements Phase 1: Agent abstraction layer with multiple model support

Features:
- Abstract AgentInterface base class with standard contract
- LoremTesterAgent: Full-featured mock agent with realistic behavior
  - Configurable reasoning effort levels (none to xhigh)
  - Random tool/function call generation
  - Error triggers for testing (rate_limit, context_overflow)
  - Temperature-based response variation
- TatlockAgent: Placeholder for future PydanticAI integration
- ModelRegistry: Centralized model management and discovery

Testing:
- 9 unit tests for lorem-tester agent behavior
- 9 unit tests for registry operations
- Coverage: Agent abstraction fully tested

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/agents/__init__.py

@@ -0,0 +1,11 @@
+"""
+Agent implementations for different models.
+
+This module provides the abstraction layer between the Responses API
+and the underlying LLM implementations (PydanticAI, Ollama, etc.).
+"""
+
+from src.agents.base import AgentInterface
+from src.agents.registry import ModelRegistry
+
+__all__ = ["AgentInterface", "ModelRegistry"]