cache/generated holds provider-generated images/videos that are now
deliberately never swept (they are the only copy of the user's output).
_KEPT_CACHE_SUBDIRS did not list it, so `hermes backup` treated it as a
regenerable cache and dropped it — for the root home and profile homes.
Add GENERATED_SUBDIR (stdlib-only agent.provider_media) to the kept set
and pin both locations in the existing cache-pruning backup test.
0ecc5c644a