Fix dead checkpoint_hub reference in .gitignore, point to real archive

The .gitignore comment referenced `python -m src.utils.checkpoint_hub
download --checkpoint <name>` as if it existed -- it never did, so every
run checkpoint (V15-V23, the ones CLAUDE.md's results sections cite as
evidence) lived only on local disk with zero backup. Archived the 6
canonical runs to https://huggingface.co/geestaltt/3-adic-vae-checkpoints
(checkpoint.pt + config.yaml + results.json each) before doing any local
cleanup, then removed runs/visualizations/v7_large (4.2GB of regenerable
PNG/SVG/HTML, zero .pt files) and 53 crashed/empty run directories (zero
checkpoints each, ~1.2MB total) -- runs/ went from 15GB/112 dirs to
11GB/59 dirs with the archived checkpoints verified intact and zero
information loss (every deleted directory had 0 .pt files).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: gesttaltt

.gitignore

@@ -134,7 +134,12 @@ sandbox-training/
 sandbox/
 
 # Large model checkpoints (stored on Hugging Face Hub)
-# Use: python -m src.utils.checkpoint_hub download --checkpoint <name>
+# The `checkpoint_hub` download tool this comment used to reference was never
+# actually built -- checkpoints lived only on local disk with no backup until
+# 2026-07-18. Canonical run checkpoints (V15-V23, the ones CLAUDE.md's
+# results sections cite) are now archived at
+# https://huggingface.co/geestaltt/3-adic-vae-checkpoints (download manually,
+# e.g. via huggingface_hub.hf_hub_download -- no CLI wrapper exists yet).
 **/*.pt
 *.7z
 /visualizations/