Can an extension participate in core Spec Kit commands?

[Ghazgkull]

Is there any way for an extension to participate in the core Spec Kit commands (e.g. plan/tasks/implement)?

I'm working on an extension which adds a command that writes markdown files to the repository. If those files exist, I want the core commands to be aware of them. This could be accomplished by simply appending a few lines of instruction to the built-in command templates, but I haven't found a way to do this.