How does the plugin system work in kak? I see the author has a plugin built as a CLI app binary, how does that interface with kak?

Also curious what's the capability of kak's plugin system. With vim you can almost do anything with vimscript, even create any UI by creating new buffers + setline(). What does kak provide for customization / scripting?

Kakoune doesn't have exactly a plugin system, but a module one. There are however various plugin managers.

Its customisation is obtained by exposing an interface to the shell with various parts of its functionality.

Can you compose grep to only search within specific scopes like "don't search in comments"?