ci: fix pipx setup for published smoke

[cafitac]

Summary

• Fixes the published install smoke workflow's pipx setup after the v0.1.19 publish run exposed a ModuleNotFoundError: No module named 'pipx' from the --user installed console script.
• Installs pipx into the active Python environment and exposes a small pipx wrapper that calls pipx.main with the same interpreter.
• Adds a regression test so both the manual smoke workflow and publish workflow keep the wrapper setup.

Verification

• uv run pytest tests/test_published_install_smoke.py -q -> 11 passed
• temp wrapper + uv run python scripts/smoke_published_install.py --version 0.1.19 --attempts 1 --delay-seconds 0 --timeout 240 -> npm registry/npx/npm exec/uvx/pipx passed
• uv run pytest tests/ -q -> 145 passed
• uv run python scripts/check_release_metadata.py -> OK
• uv run python scripts/smoke_release_readiness.py -> OK
• npm pack --dry-run -> OK
• git diff --check -> OK

Context

• PR ci: add published install smoke matrix #13 was merged and published npm/PyPI 0.1.19 successfully, but the new post-publish smoke job failed before GitHub Release creation because the workflow's pipx console script could not import the pipx module.