Use it now
Plugins can now register and later unregister their own agent definitions dynamically
What
Plugins can now register custom agent definitions (subagents, i.e. specialized helper agents a plugin provides) under a composite key combining the plugin's name and the agent's name. Plugins can also forget these registrations later, which triggers the agent list to refresh automatically.
Why
This lets plugins add and remove their own agents dynamically, for example if a plugin is disabled or reloaded, without requiring a restart to update the list of available agents.