Only require JIXIA_PATH for the subcommand that uses it

[Gabrielebattimelli]

Why

database/__main__.py read os.environ["JIXIA_PATH"] at import time, so schema, informal, and vector-db all died with KeyError: 'JIXIA_PATH' — none of which shell out to the jixia binary. Only the jixia subcommand needs it.

This blocked running any of those commands standalone (it came up while re-embedding the ChromaDB index by hand).

What

• Guard the import-time assignment.
• Have the jixia subcommand validate the variable itself, with a message naming what to set it to rather than a bare KeyError.

Verified

old: KeyError 'JIXIA_PATH'   <- broke schema/informal/vector-db
new: import-time guard passed cleanly

jixia subcommand -> SystemExit: JIXIA_PATH is not set.
                    It must point at the jixia binary, e.g. jixia/.lake/build/bin/jixia