Dagger Test fn: replace inline docker run asserts in CI

[haydenk]

Parent

Part of #14

What to build

Add Test(ctx, source *dagger.Directory, distro, version string) (string, error) to .dagger/main.go that builds the per-distro image and asserts:

• ansible --version
• python3 --version
• /etc/ansible/hosts exists
• uv --version

Replace ci.yml's test job's docker run --rm ... steps with dagger call test --distro=... --version=....

Acceptance criteria

• dagger call test --source=. --distro=debian --version=bookworm-slim runs locally and exits 0
• ci.yml test job uses dagger; no inline docker run asserts
• All six distros pass on master
• PAT-less

Blocked by

None — can start immediately.