Defines activities that workflows can call.

defmodule MyApp.Activities.Payment do
  use Temporalex.Activity

  defactivity charge(amount), timeout: 30_000 do
    Stripe.charge(amount)
  end
end

Each defactivity generates two functions:

1. charge(amount) — dispatch function called from workflow code. Sends {:execute_activity, type, input, opts} to the executor.

2. __charge__/1 — implementation function called by the server when an activity task arrives. Public for direct use in tests.

The module also generates __temporal_activities__/0 returning [{name, opts}] for registration.