Make Slurm epilog cleanup job-scoped

Track a focused follow-up for the existing Slurm job-end cleanup behavior.

40-lastuserjob-processes terminates processes by user, 42-lastuserjob-cleanup removes files by user ownership, and 50-lastuserjob-all-enroot-dirs removes configured Enroot paths. Those scopes do not necessarily belong only to the job that just ended.

Acceptance criteria:

• Limit cleanup to processes and paths owned by the ending job.
• Preserve unrelated login sessions, concurrent or suspended jobs, and shared paths used on other nodes.
• Add regression tests for missing/erroring ownership information and overlapping workloads.
• Preserve useful behavior; do not broadly disable tuning, SSH handling, or cleanup as a shortcut. Any necessary behavior change should be reviewed explicitly.

Related: #1391 and #1404. Historical #1315 concerns PID parsing; this follow-up concerns cleanup scope.

Keep this as a separate backlog item, not a prerequisite for unrelated fixes in the current PR queue.

Start by reading the 40-lastuserjob-processes, 42-lastuserjob-cleanup, and 50-lastuserjob-all-enroot-dirs scripts, then review related issues #1391 and #1404. Add regression coverage for missing or failing ownership data and overlapping workloads, and verify that cleanup affects only the ending job while preserving unrelated sessions, jobs, and shared paths.