Follow-ups from #881: surface and bound the connection temp sweep #884

Follow-ups from the review of #881 (non-blocking), tracked here as one issue:

1. Surface the cleanup report. office kill lists host records through the helper that also sweeps stale connection temp files, but it ignores the entries the sweep could not inspect or remove (tempCleanupLeft). The fix prints those entries as a warning with the path and reason, so a temp file that cannot be cleaned up is visible to the operator.
2. Bound and test the sweep's cost and error paths. Each record write and host-record listing scans the whole connections directory. The fix measures that against a large directory, bounds it if needed, and adds a real-file test for the listing's sweep and for a directory read error.

Refs #825, #881.