test(goal): make unreadable archive fixture portable

Problem:
The explicit legacy archive fail-closed test passed on Unix but failed on Windows because chmod zero does not make a file unreadable there.

Root cause:
The fixture relied on Unix permission semantics instead of creating a platform-independent archive read failure.

Fix:
Replace task_spec.json with a directory so archive decoding fails deterministically on every supported platform.

Verification:
go test ./internal/control -count=1
GOOS=windows GOARCH=amd64 CGO_ENABLED=0 go test -c ./internal/control

Author: SivanCola

internal/control/goal_legacy_restore_test.go

@@ -595,17 +595,16 @@ func TestMissingLegacyGoalCommandDoesNotStartProviderTurn(t *testing.T) {
 }
 
 func TestUnreadableExplicitLegacyArchiveBlocks(t *testing.T) {
-	if os.Geteuid() == 0 {
-		t.Skip("root can bypass archive file permissions")
-	}
 	root := t.TempDir()
 	const taskID = "unreadable-explicit-archive"
 	taskRoot := writeLegacyGoalArchive(t, root, taskID, "never run an unreadable archive")
 	specPath := filepath.Join(taskRoot, "state", "task_spec.json")
-	if err := os.Chmod(specPath, 0); err != nil {
+	if err := os.Remove(specPath); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Mkdir(specPath, 0o755); err != nil {
 		t.Fatal(err)
 	}
-	t.Cleanup(func() { _ = os.Chmod(specPath, 0o644) })
 	c := New(Options{WorkspaceRoot: root})
 	t.Cleanup(c.Close)