Ship default logging via a LogSubscriber

Both downstream applications wrote their own `perform.hot_cell` subscriber to get a log line per call, and neither should have had to.

Ship a default `ActiveSupport::LogSubscriber` in `hotcell-client`, attached by the railtie.

Placeholder for a future conversation — design not settled.

Start by reading the railtie and the existing downstream `perform.hot_cell` subscriber examples, then review how `ActiveSupport::LogSubscriber` attaches to Rails notifications. The design is explicitly unsettled, so done cannot be defined until the default logging behavior and attachment point are agreed.

Written by the indexing model from the issue text.