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>

tests/test_rate_limiter.py

@@ -1,7 +1,5 @@
 """Tests for Rate Limiter (core/rate_limiter.py)."""
 
-import time
-
 import pytest
 
 from core.rate_limiter import ClientRateLimitState, RateLimitConfig, RateLimiter, TokenBucket