feat: sync only community pull requests to Yunxiao

[syzsunshine219]

What changed

• Sync only community-authored pull requests to Yunxiao.
• Skip internal pull requests with GitHub author associations OWNER, MEMBER, or COLLABORATOR, plus unknown associations.
• Keep Issue synchronization unchanged.
• Apply the same filter to real-time events and backfill runs.

Validation

• python3 -m unittest scripts/test_yunxiao_github_sync.py (9 passed)
• git diff --check
• Project Harness business checks passed; repository baseline full_lint and full_build remain failing.