fix(core): close imported-session stores before the temp dirs are removed

The session-import tests opened a SqliteSessionStore per case and never
closed it, so afterEach's rmSync ran against a directory still holding an
open SQLite file. POSIX allows that; Windows does not, and all seven
persisting cases failed the sdk-publish Windows job with EPERM on the
cline-db-* temp dir.

Route every store through a sessionStore() helper that registers it for
close, and close them before removing the dirs.