Each behave-parallel worker previously resolved to the same file-based
SQLite database (cleveragents.db or .cleveragents/db.sqlite) because
before_scenario removed the CLEVERAGENTS_DATABASE_URL env var and the
fallback paths are shared across processes. Under parallel execution
this caused intermittent 'database is locked' and duplicate-project
errors—most visibly in the coverage_report nox session.
Replace the env-var removal in before_scenario with unique temp database
paths via tempfile.mktemp(), giving every scenario its own isolated
database file. Temp files are cleaned up in after_scenario.
Also fix cli_streaming_steps.py where a Background + duplicate Given
sequence triggered a duplicate project name collision, and update
coverage_boost_steps.py so Settings-default assertions explicitly clear
the env vars before testing pydantic defaults.
brent.edwards