Remove the cross-version heuristic from _strip_mismatched_site_packages:
the subprocess env builder cannot know which Python version a child will
run, so judging user PYTHONPATH entries against the backend interpreter's
version deletes legitimate paths meant for a different child Python
(e.g. /custom/lib/python3.13/site-packages while Hermes runs 3.11).
Also fix over-strip: entries merely containing a pythonX.Y path component
(e.g. /opt/tools/python3.13/bin) were stripped even though they are not
site-packages. Hermes-owned entries (repo root, own venv site-packages)
are now identified by path ownership, not by version.
Regression tests cover both cases; user paths with any pythonX.Y
component are preserved.
ced80b2a20