chore(tests): ignore tests/ 目录 + 同步现有测试到新 skill_archive 签名

- .gitignore: 新增 offline-backend-20260512/backend/tests/* 规则,
  让后续新增的测试文件不再误进入版本控制
- test_skills_multitenancy.py: 同步上次 feat(skills) 提交里
  skill_archive(name, reason) 必填 reason 的签名变更

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -77,3 +77,4 @@ offline-backend-20260512/backend/skill_evolution.json
 skills/
 hermes-agent-main/
 .gitignore
+offline-backend-20260512/backend/tests/*