Stream run logs over the runner's SSH channel

Stream run logs over the runner's SSH channel

The runner wrote the log to a temp file and uploaded it as one LFS object
when the job ended, so a running job had no log to read and a log was stored
like a shared artifact even though it belongs to one attempt.

The runner sends 128 KiB chunks over the same runner channel that holds the
lease. The server appends them to data/logs/<repo>/<run>/<attempt>.log and
records the size while the run is still running. The finish record carries
the final size; the server checks the file length, syncs it, and stores the
result. A partial or failed stream leaves no log on the record.

Logs are no longer LFS objects: they are not shared, not content-addressed,
and the web serves the file directly, with range and step slices. GC no
longer treats them as LFS roots.