Make the superproject selectable via the header

The header stands for the root repository: it is selected on startup,
shows the root's own state word, and the detail pane shows its state
like a submodule's (HEAD vs. upstream, changes, unpushed, leftover
submodule clones as untracked repositories). Submodules are ignored in
the root's changes; their rows show them.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/submodule/status_test.go

@@ -78,3 +78,14 @@ func TestHiddenIssuesCountLoadErrors(t *testing.T) {
 		t.Errorf("got %+v; want 1 danger", got)
 	}
 }
+
+func TestRootIssueWording(t *testing.T) {
+	st := &Status{Root: true}
+	got := st.Issues()
+	if len(got) != 1 || got[0].Kind != IssueUnpushed || got[0].Level != LevelDanger || got[0].Detail != "HEAD not pushed to any remote" {
+		t.Errorf("Issues() = %+v", got)
+	}
+	if !(&Submodule{}).IsRoot() || (&Submodule{Path: "a"}).IsRoot() {
+		t.Error("IsRoot should be true exactly for the empty path")
+	}
+}