Dev deploys: SKIP_DEV_TESTS flag and a rollback button #502

What: repo variable SKIP_DEV_TESTS=true skips the dev smoke and functional gates and shows a green "Tests skipped (SKIP_DEV_TESTS)" job; promote still marks the release last-known-good. main ignores it. Run workflow on dev with rollback ticked switches back to the previous release and records it as last-known-good.
Why it matters: dev deploys keep going while the test owner is away, with a one-click way back.
Evidence: flag-off deploy, run 37940563887: green, tests ran. Flag on since 2026-10-09; the flag-on path and the button haven't been exercised yet.
Fix: #501. Off: gh variable delete SKIP_DEV_TESTS -R CivicDataLab/DataSpaceFrontend.