Having read your explanation of channels/tracks - I get it, you have to serialize your storage into something and it's neat that it's representable through url. I was a little confused at first. I agree with others that the organization seems a little bit too forceful or opinionated for the primary use case of a "notes app" which is jotting down a quick note.

Maybe you could have an "unsorted" channel by default, and then split out the context dropdown into little icons? Simplify the work flow to be: 1. Click "new markdown" button, 2. Write some markdown. 3. Save.

If you defer asking the user to name a file up front, you can auto generate a filename based on the contents of the document (like parse 1st line for # My Title -> my-title.md). The only sacrifice you pay is making them click "save" the first time; but IMO that is a lot less friction than having to name documents before I even start writing. Plus even if your parsing fails - you're no worse off than where you started by asking the user for a filename.