Gitignore runtime artifacts in test fixtures journal
Add .gitignore to tests/fixtures/journal/ to prevent stats cache,
dream markers, task logs, and port files from being tracked. Untrack
task_log.txt (unused by any test). Remove redundant module-level
os.environ["JOURNAL_PATH"] overrides from three test files — the
conftest autouse fixture already handles this.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>