Shared runner contract test

Run the complete shared-runner contract:

bash actions/test.runner-contract.sh

It checks the shared privilege/staging API, GPU/NVIDIA/NVLink and LLM-host integration, clean root environment, single bootstrap fetch, and documented legacy-runner inventory. It then executes the shell-only staging and sudo mock suites.

The action does not call real sudo, access the network, install software, or run Ansible.