fix(ci): fix black/ruff failures, add CoC and PR template

- Fix Python formatting (sync no longer strips blank lines from .py files)
- Remove test_community_build.py (tests private sync module)
- Fix ruff warnings in test files
- Add CODE_OF_CONDUCT.md
- Add .github/PULL_REQUEST_TEMPLATE.md

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

core/site_registry.py

@@ -12,6 +12,7 @@ from typing import Any
 
 logger = logging.getLogger(__name__)
 
+
 class SiteInfo:
     """Information about a single site."""
 
@@ -50,6 +51,7 @@ class SiteInfo:
             "config_keys": list(self.config.keys()),
         }
 
+
 class SiteRegistry:
     """
     Registry for managing site configurations across plugin types.
@@ -360,9 +362,11 @@ class SiteRegistry:
                 options.add(site_info.alias)
         return sorted(options)
 
+
 # Global site registry instance
 _site_registry: SiteRegistry | None = None
 
+
 def get_site_registry() -> SiteRegistry:
     """Get the global site registry instance."""
     global _site_registry