Fix Bluesky authentication to load credentials from config.yaml
- Update default_login() to use get_bluesky_config() instead of os.getenv()
- Update init_client() to load PDS_URI from config with fallback to env var
- Update all PDS_URI references to use config loader
- Remove username/password printouts for security
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Co-Authored-By: Claude <noreply@anthropic.com>