Upgrade to 1.0.0-rc878

Concerns: pushword/core

pw:image:cache sweeps the stale image scratch files it finds

Each run now deletes the *.tmp files the image writers left behind when a process was killed mid-write — over an hour old only, under media_dir and media_cache_dir — and reports Swept N stale scratch file(s), M empty. (scratch_swept / scratch_empty in --format=agent). Nothing to run; a site sweeping those files from its own deploy or cron script can drop that step.