fix: remove unused API_ENV_VARS import in client.py

Fixes ruff F401 lint failure in CI.

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

Author: derhally

client.py

@@ -17,7 +17,6 @@
 from claude_agent_sdk.types import HookContext, HookInput, HookMatcher, SyncHookJSONOutput
 from dotenv import load_dotenv
 
-from env_constants import API_ENV_VARS
 from security import (
     SENSITIVE_DIRECTORIES,
     bash_security_hook,