selfupdate: add Check to see the newest release without installing

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

selfupdate/selfupdate_test.go

@@ -135,3 +135,20 @@ func TestVersionOrdering(t *testing.T) {
 		}
 	}
 }
+
+func TestCheckReportsWithoutInstalling(t *testing.T) {
+	srv := fakeGitea(t, "v0.2.0", false, false)
+	for _, c := range []struct {
+		current string
+		newer   bool
+	}{{"v0.1.0", true}, {"v0.2.0", false}, {"v0.3.0", false}, {"dev", false}, {"v0.1.0-local", false}} {
+		cfg, exe, restarted := setup(t, srv, c.current)
+		tag, newer, err := Check(context.Background(), cfg)
+		if err != nil || tag != "v0.2.0" || newer != c.newer {
+			t.Errorf("%s: tag=%q newer=%v err=%v", c.current, tag, newer, err)
+		}
+		if b, _ := os.ReadFile(exe); string(b) != "OLD-BINARY" || *restarted != "" {
+			t.Errorf("%s: Check changed things", c.current)
+		}
+	}
+}