Twenty-five call sites told users to run
`python -c "from pm import sync_venv; sync_venv(['x'], explicit=True)"`
because `hermes pm install` only took package names. Add `--extra`
(repeatable; syncs the venv with the named extras and nothing else) and
pm.install_hint(extra), the single builder every site now uses, so the
advice stays correct when the command changes.
A cold PM runtime under allow_lazy_installs:false now reports the extra
the caller wanted and the command that provisions both, instead of a
bare "pm-runtime: not installed".
029cabb466