Cmd::Unit: added missing Hive::Unit include.
dep: added AM‽ dependency; updated README.

dep/README: added AM‽ to clone list.
dep/.gitignore: added 'am'.
Cmd: added Cmd::type_info and command_info() to unit boilerplate.
Cmd::Unit: moved invariants to UnitBase; added context notifications.

System::Context:
    made class abstract;
    added notify_complete().

Cmd::Unit:
    added notify_complete() call post-unit-commit.
Cmd::Hive::StoreAll: corrected scope for store_object().
Cmd/Data: made internal functions static.
Cmd::Unit: took UnitBase::base_notify_complete() to commit_post().
System::Context: added notify_exception().

Cmd::Unit: added notify_exception() and notify_exception_current().
Cmd: replaced custom error reporting with notify_exception_current().