ci: add a smoke test workflow for the self-hosted runner

[WhiteMuush]

Summary

• First workflow on the self-hosted runner: prints whoami and hostname to prove the runner picks up jobs under the right account.
• Triggers: pull requests to main, pushes to main, and workflow_dispatch for manual runs (useful for the benchmark).
• Pull requests from forks are skipped by an if condition: the repository is public, fork code must never run on the VM.
• Least privilege: contents read only, 5 minute timeout.