test_relation_write.py was a manual asyncio script with a hardcoded ERP5
URL and __ac cookie — it never had pytest test functions, but its name
caused pytest to import it (and exposed credentials in git). Rename to
scripts/probe_relation_write.py so it stops being collected, read auth
from the environment, and pin testpaths in pyproject.toml so a bare
`pytest` run targets only the mocked unit suite. Prep for adding the
project to Nexedi's slapos-testing CI.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>