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/oauth/pkce.py

@@ -8,6 +8,7 @@ import hashlib
 import secrets
 from typing import Literal
 
+
 def generate_code_verifier(length: int = 64) -> str:
     """
     Generate PKCE code verifier.
@@ -34,6 +35,7 @@ def generate_code_verifier(length: int = 64) -> str:
 
     return verifier
 
+
 def generate_code_challenge(code_verifier: str, method: Literal["S256"] = "S256") -> str:
     """
     Generate PKCE code challenge from verifier.
@@ -63,6 +65,7 @@ def generate_code_challenge(code_verifier: str, method: Literal["S256"] = "S256"
 
     return challenge
 
+
 def validate_code_challenge(
     code_verifier: str, code_challenge: str, method: Literal["S256"] = "S256"
 ) -> bool: