feat: event-driven thread wait coordinator - #2688

Actions cache usage is 7.51 GB of 10.00 GB (75%) across 4 entries. GitHub evicts least-recently-used entries without reporting it, so caches are about to start missing. Run `gh cache list --limit 100 --json key,sizeInBytes` to find the job writing oversized entries; a job on a Blacksmith Linux runner should not appear here at all.