Complete SAND-WP-0001: bootstrap Python dev workflow

Scaffold sandboxer package (src layout, typer CLI stub, pytest, ruff),
Makefile targets, and document install/test/lint/build commands in
AGENTS.md and stack-and-commands.md. Mark bootstrap workplan finished.

tests/test_package.py

@@ -0,0 +1,7 @@
+"""Smoke tests for the bootstrap package scaffold."""
+
+from sandboxer import __version__
+
+
+def test_version_is_semver_placeholder() -> None:
+    assert __version__ == "0.0.0"