test(ci): skip chmod-dependent tests when running as root

Docker CI containers run as root by default (golang:1.26-alpine), where
os.Chmod has no effect and permission restrictions don't apply. Four
tests were failing because they relied on chmod to force error paths:

- gradle: TestWriteVersionReadOnly
- cmd:    TestInitConfigWriteFails, TestRunLatestTagFails
- gitutil: TestLatestTagTagsIterFails (had broken skip logic that only
  fired if Chmod itself errored, which never happens as root)

Replace all four with an upfront os.Getuid() == 0 check so they are
skipped cleanly in root environments and still run (and must pass) on
non-root local development.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/gradle/gradle_test.go

@@ -124,6 +124,9 @@ func TestWriteVersionNotFound(t *testing.T) {
 }
 
 func TestWriteVersionReadOnly(t *testing.T) {
+	if os.Getuid() == 0 {
+		t.Skip("skipping: chmod restrictions do not apply when running as root")
+	}
 	path := writeGradle(t, gradleKotlin)
 	os.Chmod(path, 0444)
 	defer os.Chmod(path, 0644)