Agent Skills: run-roachtest

Run a single CockroachDB roachtest end-to-end: pick local vs. user's GCE worker, launch detached on worker via tmux + `roachstress.sh` with long-poll done-notification and tail. Use whenever user asks to run/stress/kick off a roachtest, or just modified one and next step is running it. Single test + single iteration only; nightly loops belong elsewhere.