style: clear whitespace, unused-import and blank-line lint

Strip trailing whitespace, drop unused imports/variables/global statements,
split multi-imports, fix E3xx/E1xx spacing and E712 comparisons. No behaviour
change; only E501 long lines remain in flake8.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

hbd/client/config.py

@@ -2,7 +2,6 @@
 
 import logging
 import os
-import logging
 
 logger = logging.getLogger(__name__)
 
@@ -37,10 +36,10 @@ def load_config(path=None):
     """Load configuration from a YAML file and merge with client defaults.
 
     If YAML is not available or the file does not exist, defaults are returned.
-    
+
     Args:
         path: Path to YAML config file (default: ~/.hbc.yaml)
-        
+
     Returns:
         Dictionary with configuration
     """