hpc init: generate exhaustive template with optional fields commented out

[ultimatile]

Currently hpc init generates a minimal template with only Slurm defaults. This could be improved to:

• Emit all configurable fields exhaustively
• Comment out optional fields with their default values so users can discover them without reading docs
• Add --pjm flag to generate a PJM-oriented template

This requires replacing tomli_w.dump() with a string-based template since tomli_w does not support comments.