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/agents/prompt_loader.py

@@ -10,6 +10,17 @@ from typing import Any, Dict, Optional
 
 import yaml
 
+# Singleton instance
+_prompt_loader_instance: Optional["PromptLoader"] = None
+
+
+def get_prompt_loader() -> "PromptLoader":
+    """Get the singleton PromptLoader instance."""
+    global _prompt_loader_instance
+    if _prompt_loader_instance is None:
+        _prompt_loader_instance = PromptLoader()
+    return _prompt_loader_instance
+
 
 class PromptLoader:
     """Unified Prompt loader"""