fix(monopam): detect repos with no upstream tracking branch

ahead_behind in ocaml-git returned {ahead=0; behind=0} when the remote
ref didn't exist, making never-pushed repos appear fully synced. Now
returns None, and status shows these as "remote:new" so push correctly
identifies them as needing upstream push.

lib/repository.ml

···
   | Some local_head -> (
       let remote_ref = "refs/remotes/" ^ remote ^ "/" ^ branch in
       match read_ref t remote_ref with
-      | None -> Some { ahead = 0; behind = 0 }
+      | None -> None
       | Some remote_head ->
           if Hash.equal local_head remote_head then
             Some { ahead = 0; behind = 0 }