Implement sjnetpkg 0.1.0, a Git-based package manager

Packages are ordinary Git repositories carrying a sjnetpkg.toml, so sjnetpkg
only needs to resolve, clone, track and run them instead of inventing an
archive format or build system. This adds the v0.1 CLI (install from registry
or Git URL, remove, list/inspect, registry update, run) on top of the Python
standard library, together with a test suite.

🤖 Generated with Codebuff
Co-Authored-By: Codebuff <noreply@codebuff.com>

tests/conftest.py

@@ -0,0 +1,16 @@
+"""Test bootstrap: make the ``src`` layout importable.
+
+This is used by pytest.  When running with the standard library test runner,
+pass the source tree explicitly, e.g.::
+
+    PYTHONPATH=src python3 -m unittest discover -s tests -t tests
+"""
+
+from __future__ import annotations
+
+import sys
+from pathlib import Path
+
+_SRC = Path(__file__).resolve().parents[1] / "src"
+if str(_SRC) not in sys.path:
+    sys.path.insert(0, str(_SRC))