Add 36 test cases covering:
- Path traversal and symlink attack prevention in archive extraction
- SkillHubClient error handling (401/403/404/network) for all endpoints
- Inventory store concurrent writes and stale lock recovery
- Config store read/write round-trip
- Platform utilities (package-manager, updater, paths)
- Output formatting (printResult, humanize)
Tests use cross-platform commands (node) instead of shell builtins
for CI compatibility across macOS/Linux/Windows.
