feat(userland): set PYTHONHOME so Python finds its stdlib

[FlareCoding]

Summary

• Python could not import its shipped standard library because getpath's landmark search failed to locate the install prefix, leaving /usr/lib/python3.12 off sys.path.
• init now exports PYTHONHOME=/usr, Python's designed mechanism for a fixed install location, so every interpreter resolves its stdlib deterministically without relying on executable-path introspection.

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