feat(hpc): route caches to $VSC_SCRATCH, outputs to $VSC_DATA

- 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)

scripts/hpc/install.sh

@@ -6,22 +6,40 @@
 #   bash scripts/hpc/install.sh
 #
 # Prerequisites:
-#   - Connected to VSC via the web portal (HPC Login → Shell tmux)
-#   - Project cloned to $VSC_DATA or $VSC_HOME
-#   - Run from the project root directory
+#   - Connected to VSC via the web portal (HPC Login → Interactive Apps > Shell tmux)
+#   - Set cluster to "donphan (interactive/debug)"
+#   - Project cloned into $VSC_HOME (e.g. git clone <repo> && cd <repo>)
 
 set -euo pipefail
 
+# ---------------------------------------------------------------------------
+# Redirect caches away from $VSC_HOME (very limited at ~3 GB) to scratch.
+# This must be done before any pip/module activity.
+# ---------------------------------------------------------------------------
+export XDG_CACHE_HOME="$VSC_SCRATCH/.cache"
+export UV_CACHE_DIR="$VSC_SCRATCH/.cache/uv"
+export MPLCONFIGDIR="$VSC_SCRATCH/.config/matplotlib"
+mkdir -p "$XDG_CACHE_HOME" "$UV_CACHE_DIR" "$MPLCONFIGDIR"
+
+echo "==> Loading vsc-venv module..."
 module load vsc-venv
 
+echo "==> Activating virtual environment (created per-cluster in \$VSC_DATA)..."
+# vsc-venv transparently creates and manages a per-cluster venv in $VSC_DATA,
+# loading the modules listed in env/hpc/modules.txt and pip-installing anything
+# in env/hpc/requirements.txt that is not already satisfied.
 source vsc-venv --activate \
     --modules env/hpc/modules.txt \
     --requirements env/hpc/requirements.txt
 
+echo "==> Registering Jupyter kernel..."
 python -m ipykernel install --user --name="sel3_${VSC_INSTITUTE_CLUSTER}" \
     --display-name "SEL3 (${VSC_INSTITUTE_CLUSTER})"
 
 echo ""
 echo "✅ Environment ready. Kernel: sel3_${VSC_INSTITUTE_CLUSTER}"
-echo "   Activate in future sessions with:"
+echo "   To activate in future sessions:"
 echo "     module load vsc-venv && source vsc-venv --activate --modules env/hpc/modules.txt"
+echo ""
+echo "   Remember to also set cache dirs in any interactive session:"
+echo "     export XDG_CACHE_HOME=\$VSC_SCRATCH/.cache"