Ability to view open sessions

It would be useful to navigate to active mindstream sessions, as suggested in #18. With the upcoming changes in #24 to remove global state and rely exclusively on git branches to determine which buffers correspond to Mindstream "streams," (i.e., simply those that have a Git branch name starting with `mindstream-`), we could determine this by doing something like:

1. Use `mindstream--for-all-buffers` together with `mindstream-git-branch-name` to map open buffers to their git branch name (or better, simply map to the result of `mindstream-stream-p`).
2. Group these buffers by their git repo name (`mindstream-backend-root`).
3. Show these in a completion menu.

We could potentially provide variants of this feature that show active (a) anonymous sessions, or (b) saved sessions (those in `mindstream-save-session-path`), or (c) all sessions.

Start by reading the proposed changes in #24 and the related suggestion in #18. Inspect mindstream--for-all-buffers, mindstream-git-branch-name, mindstream-stream-p, mindstream-backend-root, and mindstream-save-session-path to understand the available session data. Done means active sessions can be grouped by repository and selected from a completion menu, with the anonymous, saved, or all-session scope resolved.

Written by the indexing model from the issue text.