* Restore sandbox test performance
- Re-enable secure native compiler caching for repo-local test artifacts.
- Isolate strict cache coverage while keeping it off the sandbox critical path.
* fix: respect sandbox test runtime limits
- Add a custom Node 24 test image with tenant-neutral VCR configuration.
- Shard differential suites across disposable Sandboxes while preserving host-specific coverage.
- Load local agent credentials safely and document the validation workflow.