Venv Bootstrap Skill
Resolves the interpreter path inside a local virtualenv, then upgrades pip in it.
_venv_python's own parameter is named venv_dir; main's own local variable —
computed deterministically from __file__, never from caller input — is ALSO named
venv_dir, purely by coincidence. Taint tracking must not conflate the two just
because they share a name. Must not be a CRITICAL vet failure.