Suggesting to ...:

• make the clinic tool a well-behaved command-line citizen; write to stderr, not stdout, on error (partly depends on Rework Argument Clinic error handling #107468)
• put the whole CLI in main, making it easier to mock the tool in the test suite (currently we use subprocesses, which is slow, and requires several lines of boiler-plate test setup code)
• Introduce CLIError and use it for usage errors Used ArgumentParser.error instead.

Linked PRs

• gh-107467: Restructure Argument Clinic command-line interface #107469