Add venv interpreter config and gitignore

VS Code workspace settings point to project-local .venv so the correct
Python interpreter is auto-selected on open.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.vscode/settings.json

@@ -0,0 +1,5 @@
+{
+  "python.defaultInterpreterPath": "${workspaceFolder}/.venv/bin/python3",
+  "python.terminal.activateEnvironment": true,
+  "jupyter.notebookFileRoot": "${workspaceFolder}"
+}