Confirm a run's own VMs in getting-started step 5

Step 5 of the getting-started guide told a reader that gcloud compute instances list --filter=labels.purpose=gitlab-simulation should come back empty after teardown. That filter lists every simulation VM in the project, teammates' included, so in a shared project it never comes back empty and sends the reader hunting for a leak that is not theirs.

The step now filters on labels.owner=<owner label>, the same filter SKILL.md and gce/operations.md use for your own run, and one sentence points to operations.md for where the owner label comes from. A sweep of docs, README and skills found no other claim that the purpose filter returns nothing. A CHANGELOG entry is under Unreleased, Fixed.