feat(agent): add screen blackout feature

- Physical displays blanked via overlay
- Remote video and input keep working
- Exclude overlay from screen capture
- Add blackout API and UI toggle
- Wire flag through capture protocol

lib/blackout/blackout_test.go

@@ -0,0 +1,10 @@
+package blackout
+
+import "testing"
+
+func TestDisabledByDefault(t *testing.T) {
+	t.Parallel()
+	if Enabled() {
+		t.Fatal("blackout should start off")
+	}
+}