- Resolve SHELL to nix store bash path (was /bin/bash which doesn't exist in sandbox)
- Inject clean SANDBOX_PATH via makeBinPath (was leaking entire host PATH)
- Add --shell flag to drop into sandboxed bash for manual verification
- Use nix-claude-code flake for claude-code binary instead of host PATH discovery
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>