ci: janitor cancels jobs no runner can ever take

windy-pro alone left ~4 jobs per run waiting forever (build-electron on
macos/windows/ubuntu-latest, deploy if:false): Gitea evaluates job if:
only at pick time, the labels do not exist here, and waiting jobs are
invisible in /actions/tasks. 37 such jobs across 10 runs today. After
30 min they are cancelled and the run status recomputed. Runs each sync,
non-fatal.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/sync_from_github.sh

@@ -80,6 +80,10 @@ for r in $REPOS; do
   fi
 done
 
+# Jobs that name labels no runner has (ubuntu/macos/windows-latest) would wait
+# forever and invisibly; cancel them after 30 min. Never fails the sync.
+bash "$(dirname "$0")/cancel_unrunnable.sh" || log "janitor failed (non-fatal)"
+
 # Private repos can't run GitHub Actions; mirror their open PRs here so CI
 # fires, and post the verdicts back to GitHub as commit statuses.
 if ! python3 "$(dirname "$0")/pr_status_bridge.py"; then