Match comment style from previous cleanup commit:
- No section divider bars, no echo progress lines, no prose comments
on obvious commands
- Replace XDG_CACHE_HOME with explicit PIP_CACHE_DIR and UV_CACHE_DIR
- Drop MPLCONFIGDIR (matplotlib cache is negligible in size)
- Install: redirect XDG_CACHE_HOME, UV_CACHE_DIR, MPLCONFIGDIR to
$VSC_SCRATCH before any pip/module activity ($VSC_HOME is only ~3 GB)
- train.pbs:
- Set same cache env vars at job start
- Write run outputs and WandB cache to $VSC_SCRATCH during the run (fast I/O)
- Copy results to $VSC_DATA/runs/<jobid>/ on completion for persistence
- Redirect PBS stdout/stderr to $VSC_DATA/runs/<jobid>/job.{out,err}
- Add cd $PBS_O_WORKDIR as first step (HPC best practice)
