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_proposals.py

@@ -10,16 +10,18 @@ import respx
 from steward.config import Settings
 from steward.llm.client import LLMClient
 from steward.proposals.generator import Proposal, ProposalGenerator
+from tests.conftest import make_settings
 
 
 def _make_settings(**kwargs) -> Settings:
+    """Wrapper for test compatibility."""
     defaults = dict(
         openai_api_key="test-key",
         analysis_target_url="https://example.com/api/status",
         analysis_target_api_key="secret",
     )
     defaults.update(kwargs)
-    return Settings(**defaults)
+    return make_settings(**defaults)
 
 
 @pytest.fixture
@@ -94,6 +96,7 @@ async def test_fetch_sends_bearer_token(settings, mock_llm):
 def test_proposal_format_for_telegram():
     """format_for_telegram() should contain the source URL and body."""
     from datetime import datetime
+
     p = Proposal(
         title="Test",
         body="**Summary:** All good.",