fix: remove Python/uv from CI and fix smoke test SIGTERM failure

The agent is no longer a pnpm workspace member, so setup-python and
setup-uv are unnecessary in CI. Their post-job cache step failed because
uv never ran. Also drop Python from the smoke matrix (8→4 jobs) and
add `wait` after killing the frontend process to suppress the SIGTERM
exit code that was failing the smoke step.