Plugin

You might say

AI plugins, hooks, and Skills all look like extensions. Which one actually fits this need?

A plugin is a distributable software component installed into a host application to add capabilities.For example, an editor plugin can add a sidebar, shortcuts, code completion, or diff previews. Other plugins may only add commands or service integrations. A plugin does not require a UI, and it is not the same as a Skill task method or a Hook event script.
PluginIDE Extension
Further reading