[User Story] As a Challenges Mod, I want a command that stores challenge submissions so that I can easily view all submissions

**Given...**
An active challenge is open for submissions

**When...**
The current user sends "c!submit "

**Then...**
Store the GitHub URL with the current user's ID

No files or tests are named in the issue. Start by locating the C# Discord command handler for `c!submit` and the existing persistence or challenge-management entry points; done means an active challenge submission stores the supplied GitHub URL with the current user's ID and can support viewing submissions.

Written by the indexing model from the issue text.