Block manual GitLab CI on main

[yoshifuminakamura]

Summary

• reject GitLab Manual CI runs that target the production main branch by name
• document that Manual CI is for development and release-candidate validation
• keep production main measurements on the Portal-managed trigger path with explicit scope and RESULT_SERVER variables

Tests

• python3 -c 'import yaml; yaml.safe_load(open(".github/workflows/gitlab-manual-ci.yml", encoding="utf-8"))'
• python3 scripts/tests/check_text_integrity.py
• python3 scripts/tests/check_commit_messages.py
• git diff --check