Replace the broad `~/.cow` directory check with a regex that matches
only the credential file path (`\.cow[/\\]\.env`), so legitimate access
to other `~/.cow/` subdirectories (e.g. skills) is no longer blocked.
Drop the incomplete env/printenv blocking rule per reviewer feedback.
Rewrite test_invariant_bash.py to use the correct Bash().execute()
API and cover both the blocked and allowed cases.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
