Ubuntu runner, Python 3.11 and 3.13 (stdlib-only project: oldest
supported and newest current). Git identity and init.defaultBranch=main
configured for the tests that build scratch repos; everything else the
suite needs (bash, tar, rsync, stub binaries) is on the runner already.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
