The in-band sweep in _write_dm_file only runs when another DM is
written — a gateway that never sends one keeps orphans forever. Expose
the sweep as cleanup_bot_dm_cache() with the same contract as the other
cleanup_*_cache helpers (returns files removed) and wire it into the
gateway housekeeping loop on the hourly media-cache cadence. Also sweeps
legacy hermes-dm-*.txt and hermes-relay-dm-*.txt orphans in the OS temp
root.
Folded in from #92407 (mehmetkr-31), adapted to the runner-owned
cleanup design salvaged from #91902.
