Remove Z3 fixtures from repo — generated artifacts

Z3 fixtures are generated by: proof proptest specs/system policy --source z3
They don't belong in version control.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: buger

.gitignore

@@ -98,3 +98,4 @@ ci/tests/specs/policies
 tests/policy/properties/
 tests/policy/tc-*.json
 tests/policy/z3-*.json
+tests/policy/z3-fixtures/