chore(v2): establish protocol and test foundation

backend/src/backup_tool/__init__.py

@@ -0,0 +1,3 @@
+"""Backup Tool v2 package."""
+
+__version__ = "2.0.0.dev0"

backend/src/backup_tool/cli.py

@@ -0,0 +1,23 @@
+"""Minimal v2 command entry point; runtime roles arrive in M1."""
+
+from __future__ import annotations
+
+import argparse
+from collections.abc import Sequence
+
+from backup_tool import __version__
+
+
+def build_parser() -> argparse.ArgumentParser:
+    parser = argparse.ArgumentParser(prog="backup-tool")
+    parser.add_argument("--version", action="version", version=__version__)
+    return parser
+
+
+def main(argv: Sequence[str] | None = None) -> int:
+    build_parser().parse_args(argv)
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())