fix(uninstall): warn when Background Items entries remain (#940)

mole uninstall removes the app bundle, LaunchAgents/LaunchDaemons, and the
legacy login item, but it never inspected the modern Background Tasks
Management database. Apps registered as SMAppService helpers (Paste,
Hazel, etc.) kept showing up under System Settings > Login Items &
Extensions > App Background Activity until something else nudged the
database.

Apple has no public CLI to remove individual BTM records and resetbtm
clears everything with a user-trust prompt, so the safe fix is detect
and warn. After each batch, run sfltool dumpbtm once, match each
successfully uninstalled bundle id against the dump, and surface a
warning in the summary block with the exact System Settings path to
clear it manually.