chore: ignore .ipynb to keep demo/scratch notebooks out of the repo

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: claude

.gitignore

@@ -1,2 +1,5 @@
 node_modules
-.idea
+.idea
+# Demo / scratch notebooks must not be committed.
+*.ipynb
+.ipynb_checkpoints/