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>

tests/agents/__init__.py

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+"""Tests for agent implementations."""