ci: pin image via generated overlay instead of downloading kustomize

kubectl's built-in kustomize applies a temporary overlay that overrides
the image tag, removing the slow kustomize download from GitHub.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

k8s/kustomization.yaml

@@ -15,7 +15,7 @@ labels:
       app.kubernetes.io/name: personal-site
       app.kubernetes.io/managed-by: kustomize
 
-# CI pins this to sha-<commit> with `kustomize edit set image` before applying.
+# CI pins the tag to sha-<commit> via a generated overlay (see deploy.yml).
 images:
   - name: jesseinit-portfolio
     newName: tea.jesseinit.dev/jesseinit/personal-site