chore: scaffold project files

IDEA.md, SESSION.md, DECISIONS.md, CLAUDE.md, .gitignore.
Next session: invoke writing-plans skill against design spec.

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

.gitignore

@@ -0,0 +1,9 @@
+__pycache__/
+*.pyc
+.pytest_cache/
+*.egg-info/
+venv/
+.env
+assets/
+samples/
+.backup/