fix: run the module when the docsgpt command is not on PATH

Refusing to write the service units when `docsgpt` is not on PATH was wrong. A
package installed in a virtualenv is runnable whether or not its console script
is on PATH, and CI runs pytest as `python -m pytest`, where argv[0] is a module
file: the refusal failed thirteen native tests there.

The launcher now prefers the command on PATH, resolved to an absolute path
since PATH can hold relative entries, then an argv[0] that can be executed, and
otherwise this interpreter with `-m docsgpt`, which works wherever the package
is importable. `python -m docsgpt` became an entrypoint of its own and has a
test that runs it.

tests/test_cli.py

@@ -34,6 +34,16 @@ class TestTopLevel:
         subprocess.run([sys.executable, "-c", code], cwd=Path(__file__).resolve().parents[1], check=True)
 
 
+class TestModuleEntrypoint:
+    def test_python_m_docsgpt_runs_the_cli(self):
+        """`python -m docsgpt` is what a native service falls back to when the script is not on PATH."""
+        result = subprocess.run(
+            [sys.executable, "-m", "docsgpt", "--version"],
+            cwd=Path(__file__).resolve().parents[1], capture_output=True, text=True, check=True,
+        )
+        assert result.stdout.strip() == f"docsgpt {__version__}"
+
+
 class TestHome:
     @staticmethod
     def _installed(monkeypatch, tmp_path):