Add .gitignore for Python artifacts and runtime data

Excludes __pycache__, compiled Python files, virtual environments,
the data/ directory (runtime-generated), and build artifacts.

https://claude.ai/code/session_01KjaNo9RXevw6x1DjJD8mj6

.gitignore

@@ -0,0 +1,19 @@
+# Python
+__pycache__/
+*.py[cod]
+*.pyo
+*.pyd
+.Python
+
+# Virtual environments
+.venv/
+venv/
+env/
+
+# Data directory (runtime files, not source)
+data/
+
+# Distribution / packaging
+*.egg-info/
+dist/
+build/