Split SKILL.md into SKILL.md (rules) + LEARNING.md (stories/mistakes) + TODO.md (next steps)"

SKILL.md: Add VRAM estimation, checkpointing, multi-session, multi-GPU lessons from CIFAR OOM

Update SKILL.md with DataLoader batch size mismatch bug and shared state pitfall

Add SKILL.md — paper reproduction skill with lessons from NSGF++ implementation