refactor: Fix code quality issues identified in analysis

1. Rename factory.py's EvoAgent data class to AgentConfig
   - Avoids naming conflict with base/evo_agent.py's EvoAgent

2. Export pipeline_runner functions in backend/core/__init__.py
   - Add create_agents, create_long_term_memory, stop_gateway

3. Consolidate PromptLoader to singleton pattern
   - Add get_prompt_loader() singleton function
   - Update all usages to use singleton

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/core/pipeline_runner.py

@@ -17,7 +17,7 @@ from typing import Any, Dict, Optional, Callable
 from backend.agents import AnalystAgent, PMAgent, RiskAgent
 from backend.agents.skills_manager import SkillsManager
 from backend.agents.toolkit_factory import create_agent_toolkit, load_agent_profiles
-from backend.agents.prompt_loader import PromptLoader
+from backend.agents.prompt_loader import get_prompt_loader
 from backend.agents.workspace_manager import WorkspaceManager
 from backend.config.constants import ANALYST_TYPES
 from backend.core.pipeline import TradingPipeline
@@ -36,7 +36,7 @@ from backend.services.storage import StorageService
 from backend.services.gateway import Gateway
 from backend.utils.settlement import SettlementCoordinator
 
-_prompt_loader = PromptLoader()
+_prompt_loader = get_prompt_loader()
 
 # Global gateway reference for cleanup
 _gateway_instance: Optional[Gateway] = None