Sets GSTACK_STATE_DIR to a temp directory at module level, so all
E2E skill runs write artifacts to the temp dir instead of the real
~/.gstack/projects/. Prevents E2E garbage from accumulating.
The plan-eng-review artifact test now uses the isolated state dir
instead of hardcoding ~/.gstack/projects/test-project.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>