Move plugin capability enumeration, loader
subscriptions, and action construction into
ScriptActionSource. Keep ActionRegistry focused
on generic action state, dispatch, and snapshots.
Use source rather than package for generic origin
metadata so future non-plugin providers share the
same interface. Action IDs and persisted
configuration remain unchanged.
Subscribe before initial enumeration so plugin
events cannot be missed between the snapshot and
callback registration. Add a focused test for
source startup.
