ci: install pipx in smoke workflow venv

[cafitac]

Summary

• Fixes the published install smoke CI pipx setup after v0.1.22 exposed that the temporary wrapper imported a non-existent pipx.main module.
• Installs pipx into the workflow's uv-managed .venv and adds .venv/bin to GITHUB_PATH, so the real pipx console script runs with the Python environment where pipx is installed.
• Updates regression tests to prevent reintroducing the broken pipx.main/pip install --user pipx setup.

Test Plan

• uvx --from pyyaml python - <<'PY' ... yaml.safe_load(...) ... PY
• uv run pytest tests/test_published_install_smoke.py -q — 11 passed
• uv run pytest tests/ -q — 145 passed
• git diff --check
• PATH="$PWD/.venv/bin:$PATH" uv run python scripts/smoke_published_install.py --version 0.1.22 --attempts 1 --delay-seconds 0 --timeout 180 — all npm/npx/npm-exec/uvx/pipx commands returned 0
• uv run python scripts/check_release_metadata.py
• uv run python scripts/smoke_release_readiness.py

Context

The failed publish run 25145720909 published npm/PyPI successfully, then failed before GitHub Release creation because published-install-smoke retried the same pipx wrapper import error 36 times:

ModuleNotFoundError: No module named 'pipx.main'