fix: restore kube-system for sealed-secrets-controller refs

Reverts docs changes from 143b2c3 that incorrectly replaced
kube-system with headlamp for sealed-secrets-controller commands.

The sealed-secrets-controller runs in kube-system, NOT headlamp.
Only the Headlamp app install namespace was changed to headlamp.

Changes:
- Revert -n headlamp → -n kube-system in all sealed-secrets-controller
  kubectl commands across all docs files
- Revert sealed-secrets-controller.kube-system DNS reference
- Revert --controller-namespace=headlamp → --controller-namespace=kube-system
- Revert 'namespace headlamp' → 'namespace kube-system' in error messages

Co-Authored-By: Paperclip <noreply@paperclip.ing>

docs/getting-started/quick-start.md

@@ -162,7 +162,7 @@ For CI/CD or offline encryption:
 
 **Check controller status**:
 ```bash
-kubectl get pods -n headlamp -l name=sealed-secrets-controller
+kubectl get pods -n kube-system -l name=sealed-secrets-controller
 ```
 
 **If not running**, install it:
@@ -197,7 +197,7 @@ rules:
 
 2. **Verify controller connectivity**:
    ```bash
-   kubectl get svc -n headlamp sealed-secrets-controller
+   kubectl get svc -n kube-system sealed-secrets-controller
    ```
 
 3. **Check browser console**: