refactor(hpc): use explicit PIP_CACHE_DIR/UV_CACHE_DIR, drop prose comments

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)

docs/HPC.md

@@ -81,9 +81,8 @@ module swap cluster/donphan
 qsub -I -l nodes=1:ppn=4 -l walltime=1:00:00
 
 # Once inside the job — redirect caches to scratch first
-export XDG_CACHE_HOME="$VSC_SCRATCH/.cache"
+export PIP_CACHE_DIR="$VSC_SCRATCH/.cache/pip"
 export UV_CACHE_DIR="$VSC_SCRATCH/.cache/uv"
-export MPLCONFIGDIR="$VSC_SCRATCH/.config/matplotlib"
 
 # Change to the project directory and activate environment
 cd "$PBS_O_WORKDIR"