fix(update-triage): treat maintainer login as additive

[Terry-Mao]

Summary

• Treat update-triage maintainer_login inputs as an additional maintainer source instead of an exclusive allowlist.
• Preserve OWNER/MEMBER/COLLABORATOR and org-member fallback maintainer detection when explicit logins are configured.
• Update aggregate triage feedback coverage for the additive behavior.

Validation

• PYTHONDONTWRITEBYTECODE=1 python3 -m unittest discover -s .github/aicodingflow-tests -p 'test_aggregate_triage_feedback.py'
• git diff --check

[github-actions]

未发现需要阻塞合并的正确性、安全性或测试覆盖问题。