Fix Ruff lint warnings (unused imports and unnecessary f-strings)

agent/display.py

@@ -6,7 +6,6 @@ Used by AIAgent._execute_tool_calls for CLI feedback.
 
 import json
 import os
-import random
 import sys
 import threading
 import time

agent/redact.py

@@ -9,7 +9,6 @@ the first 6 and last 4 characters for debuggability.
 
 import logging
 import re
-from typing import Optional
 
 logger = logging.getLogger(__name__)