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.
