[prompt files] Explore expressing MCP or extension dependencies within prompt and mode file

[aeschli]

[charris-msft]

I would like to be able to include prompt files within my mode files.

I should be able to reference the prompts via relative path in the workspace, by name (if they are in my profile), or by #fetch command so I can pull prompts from Awesome Copilot or other collections of prompts.