refactor: Split conversational.py into modular structure (fixes #63)

Split conversational.py (2271 lines) into modular files:
- tools.py: TOOL_REGISTRY, get_tool_registry(), SKILL_EMOJIS
- help.py: format_* functions for slash command help
- client.py: MiniMaxClient, SYSTEM_PROMPT, TOOLS definitions
- agent.py: ConversationalAgent class with all methods
- __init__.py: Public exports from all modules

Updated bots.py import to use new module path.
Deleted conversational.py.

src/backend/app/services/ai_agent/__init__.py

@@ -1,4 +1,29 @@
+"""AI Agent module for conversational trading."""
+
+from .agent import ConversationalAgent, get_conversational_agent
+from .client import MiniMaxClient
+from .tools import get_tool_registry, TOOL_REGISTRY
+from .help import (
+    format_tools_list,
+    format_general_help,
+    format_tool_help,
+    format_skill_acknowledgment,
+)
 from .crew import TradingCrew, get_trading_crew
 from .llm_connector import MiniMaxLLM, MiniMaxConnector
 
-__all__ = ["TradingCrew", "get_trading_crew", "MiniMaxLLM", "MiniMaxConnector"]
+__all__ = [
+    "ConversationalAgent",
+    "get_conversational_agent",
+    "MiniMaxClient",
+    "get_tool_registry",
+    "TOOL_REGISTRY",
+    "format_tools_list",
+    "format_general_help",
+    "format_tool_help",
+    "format_skill_acknowledgment",
+    "TradingCrew",
+    "get_trading_crew",
+    "MiniMaxLLM",
+    "MiniMaxConnector",
+]