Fold the remaining simplify-code reuse finding: teams' _install_hint()
duplicated lazy_deps' spec-fetch + quote + join (feature_install_command
already builds pip commands from LAZY_DEPS). Add a venv_pip=True variant
to feature_install_command — sys.executable -m pip targeting, correct in
every install layout and immune to PEP 668 — and shrink the teams helper
to a one-line call. Also gives matrix and the other platforms a shared
derived hint to adopt later. New test mutation-checked (fails when
venv_pip returns the uv form).
