Add reference image generation script and .env to gitignore

generate_references.py automates generating pure-black and pure-white
reference images via the Gemini API at multiple aspect ratios (9:16, 4:3).
Includes rate-limit retry logic and per-resolution output directories.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -25,6 +25,9 @@ Thumbs.db
 *.pkl
 !artifacts/codebook/*.pkl
 
+# Secrets
+.env
+
 # Temporary
 *.tmp
 *.log