feat: show API key SHA256 hash instead of asterisk masking

Replace the generic asterisk masking with a deterministic SHA256 hash
prefix to allow developers to verify which API key is being used while
maintaining security.

Changes:
- Add hashlib import for SHA256 support
- Create get_api_key_hash() helper function
- Update config display to show: API_KEY=sha256:<hash_prefix>...

Author: elifarley

src/core/config.py

@@ -1,5 +1,6 @@
 import os
 import sys
+import hashlib
 
 # Configuration
 class Config:
@@ -69,9 +70,15 @@ def get_custom_headers(self):
 
         return custom_headers
 
+def get_api_key_hash(api_key):
+    """Generate SHA256 hash of API key for secure display"""
+    if not api_key:
+        return "<not-set>"
+    return hashlib.sha256(api_key.encode()).hexdigest()[:8]
+
 try:
     config = Config()
-    print(f" Configuration loaded: API_KEY={'*' * 20}..., BASE_URL='{config.openai_base_url}'")
+    print(f" Configuration loaded: API_KEY=sha256:{get_api_key_hash(config.openai_api_key)}..., BASE_URL='{config.openai_base_url}'")
 except Exception as e:
     print(f"=4 Configuration Error: {e}")
     sys.exit(1)