util/: share one stdlib-only GitHub Actions helper instead of re-implementing annotations and outputs per script #2047

Is your feature request related to a problem?
The util/ scripts that #1967 and #2030 moved the workflow shell into each re-implement the same GitHub Actions plumbing. The ::warning/::error annotation writers, $GITHUB_OUTPUT and $GITHUB_STEP_SUMMARY appenders, and subprocess run-and-report wrappers are copied across apt_packages.py, classify_changes.py, discussion_report.py, leg_summary.py, pip_install.py, regenerate_samples.py, superseded.py, vendor_test_gate.py, verdict_covers_checkout.py and others.

Is a third-party tool the answer? I don't think so, for two reasons:

• pexpect drives interactive programs over a pty. None of these commands is interactive: each runs, exits, and has its output parsed, which subprocess.run already does. pexpect would add a dependency, and its spawn is unavailable on Windows (pexpect docs, "Pexpect on Windows").
• The CI-path scripts are standard library only on purpose (see the scripts section of workflows.rst). They run on the image's /usr/bin/python3, or on a leg interpreter that may be broken, 3.7, GraalPy or free-threaded, and pip_install.py is the install step. A third-party helper would have to be installed by the step it exists to run.
• Scripts that run in a controlled environment already use third-party packages where it pays: requests and lxml in ci_timing.py, psutil in vendor_test_gate.py, yaml and markdown_it in update_python_versions.py. That stays as it is.

Needs you:

• (a) Recommended: add one stdlib-only helper, util/_actions.py, with annotate(), set_output(), summary(), group() and run(). Each script imports it, and a project test pins the helper as standard library only. It is one PR per batch of scripts, with no behaviour change, and each moved script keeps its existing tests.
• (b) Leave it duplicated. Each script stays self-contained, so it can be copied or run without the others.
• (c) Adopt a third-party toolkit only for scripts off the install path. That gives two styles side by side.