fix: delete coglet-managed output files after upload
Coglet creates output files (IOBase writes, oversized JSON spills) in
/tmp/coglet/predictions/{id}/outputs/ but never deletes them. This can
cause stale-output bugs where models return outputs from previous
predictions if an overwrite fails (issue #1434).
Add a managed bool field to the FileOutput IPC protocol message so
the orchestrator knows which files are safe to delete (coglet-created)
vs user-authored Path outputs (returned by reference, must not be
deleted). The orchestrator deletes managed files immediately after
reading their bytes into memory. A backstop remove_dir_all in
remove_prediction cleans up the entire prediction directory to catch
files from aborted uploads or cancelled predictions.
Closes #1434
