Support Python 3.6 in Slurm provenance writers

[jder]

Summary

• keep the Apptainer train and eval provenance writers compatible with ORCD's host Python 3.6
• replace str.removeprefix with equivalent prefix slicing in both Slurm harnesses
• document why the older-Python-compatible form is required

Root cause

The Slurm harness writes run-provenance.json with the host python3 before launching Apptainer. ORCD currently provides Python 3.6.8 on the host, where str.removeprefix is unavailable. As a result, H200 smoke-test job 18753331 pulled the correct container successfully but exited before srun launched training.

Impact

Training and evaluation jobs can now write provenance metadata and proceed to their container launch on ORCD's host Python.

Checks

• bash -n scripts/slurm_apptainer_train.sbatch scripts/slurm_apptainer_eval.sbatch
• executed each harness's embedded provenance writer with ORCD host Python 3.6.8 and verified the expected JSON keys
• git diff --check

[jder]

I'm not sure why the default python on this host is one that was EOL in 2023 but this seems like an easy fix at least.