build_subsets.main() depends on prepare() having run, and fails with a KeyError deep in scan() otherwise

[realmarcin]

From the review of #108.

MECHS and hab_pages are declared empty at module level and filled by prepare(); only the __main__ block calls prepare() before main(). Anyone importing the module and calling main() — a test, a future driver script — gets KeyError: 'HabitatMech' from scan() rather than a message saying the checkouts were never resolved.

Fix belongs in #108: have main() call prepare() itself (it is idempotent enough for a single run, and main() is the one entry point that needs the filesystem), and keep __main__ calling main() alone.