This commit introduces:
- Flexible runner URL configuration via `SOLSTICE_RUNNER_URL(S)` for cloud-init.
- Automated detection of OS-specific runner binaries during VM boot.
- Tasks for cross-building, serving, and orchestrating Solstice runners.
- End-to-end VM build flows for Linux and Illumos environments.
- Enhanced orchestration with multi-runner HTTP serving and log streaming.
