test: update test suite for OmegaConf config system

- Add tests/conftest.py with make_settings() helper that maps legacy config
  parameter names to new nested OmegaConf structure
- Update all test files to use the conftest fixture
- All 73 tests now pass with new config system
- Maintains backward compatibility via Settings class properties

Also add AGENTS.md with comprehensive AI agent guidelines:
- Project overview and key technologies
- Directory structure reference
- Development workflow and common tasks
- Testing strategy and patterns
- CI/CD pipeline overview
- Common pitfalls and best practices
- Debugging guide for agents working on the project

tests/test_llm_client.py

@@ -6,9 +6,11 @@ import pytest
 
 from steward.config import Settings
 from steward.llm.client import LLMClient
+from tests.conftest import make_settings
 
 
 def _make_settings(**kwargs) -> Settings:
+    """Wrapper for test compatibility."""
     defaults = dict(
         telegram_bot_token="test-token",
         openai_api_key="test-key",
@@ -16,7 +18,7 @@ def _make_settings(**kwargs) -> Settings:
         openai_system_prompt="You are Steward.",
     )
     defaults.update(kwargs)
-    return Settings(**defaults)
+    return make_settings(**defaults)
 
 
 @pytest.fixture