fix: clean up oauthCredentials after migration

- Remove oauthCredentials dict after extracting api_key to avoid duplication
- Use _ for unused provider_name variable per convention

Addresses review feedback from PR #32.

nanobot/config/loader.py

@@ -95,7 +95,7 @@ def _migrate_config(data: dict) -> dict:
 
     # Extract api_key from oauthCredentials if present
     providers = data.get("providers", {})
-    for provider_name, provider_config in providers.items():
+    for _, provider_config in providers.items():
         if isinstance(provider_config, dict):
             oauth_creds = provider_config.get("oauthCredentials")
             if oauth_creds and isinstance(oauth_creds, dict):
@@ -103,5 +103,7 @@ def _migrate_config(data: dict) -> dict:
                 # Only set api_key if not already set and access_token exists
                 if access_token and not provider_config.get("api_key"):
                     provider_config["api_key"] = access_token
+                    # Clean up migrated data to avoid duplication
+                    del provider_config["oauthCredentials"]
 
     return data