Add action explanation and follow-up functionality to MachaChatSession

- Introduced `explain_action` method to provide detailed explanations for pending actions in the approval queue.
- Added `answer_action_followup` method to handle user follow-up questions regarding proposed actions.
- Updated `main` function to support discussion mode with action explanations and follow-ups.
- Refactored `conversation.py` to utilize the unified chat implementation from `chat.py`, enhancing compatibility and functionality.
- Enhanced error handling for file operations and user input validation in both new methods.

conversation.py

@@ -2,11 +2,12 @@
 """
 Macha conversation interface - legacy compatibility wrapper.
 This module now uses the unified chat.py implementation.
+All discussion functionality has been moved to chat.py.
 """
 
 # Import the unified implementation
-from chat import ask_main
+from chat import main as chat_main
 
 # Entry point
 if __name__ == "__main__":
-    ask_main()
+    chat_main()