* ci: deduplicate PR runs and serialize live E2E
* ci: preserve live E2E cleanup on supersession
* ci: harden live E2E supersession check
* ci: gate live E2E on dry-run planning
Make the dry-run result a hard prerequisite for live E2E so skip-mode changes never acquire the repository-wide slot. This intentionally trades one full dry-run duration of live startup latency for lower contention on the exclusive queue.
* ci: bound dry-run E2E planning
The dry-run job is now a hard prerequisite for live E2E. Bound its
execution so a stalled planning job cannot delay a PR verdict for the
default six-hour job limit.
* test: tighten live E2E supersession contract
