control: registry gaps hit by iOS creation screens — branch list, milestone on mr show #45

Two small read gaps hit while building the iOS create/edit screens:

• No command lists branches. mr create --source <branch> --target <branch> forces typed branch names on a phone; a picker needs repo branches <owner/name> (name, sha, default/protected flags). The web's branch dropdowns read the store directly.
• mr show does not include the milestone, so mr milestone <owner/name> <n> <title> is a blind write for any API client — set it and nothing readable changes. Add milestone to mrOut alongside the issue side, which already carries it.

Ref #11