Run the scheduled performance soak on a disposable host #86

The daily long-soak workflow is currently opt-in and runs on GitHub-hosted compute, so it has never produced the trusted self-hosted evidence described by the harness contract.

Use a disposable performance host for each scheduled or manually dispatched run:

• provision a fixed-size host only after a trusted main workflow starts;
• check out and test the exact Actions SHA;
• keep provider credentials off the test host;
• retrieve build/perf artifacts even when the harness fails;
• delete the host on success, failure, or cancellation;
• reject durations or concurrency outside bounded operating limits;
• require trusted_long_soak_v1 evidence from the two-hour default run;
• keep pull requests unable to access provisioning credentials.

After landing, run the normal two-hour soak once and verify both its evidence artifact and provider-side deletion before enabling the daily schedule.