feat: SOPS + age encrypted secrets structure

- .sops.yaml with 3 age keys (admin, dev, prod)
- infra/gitea/values/*.enc.yaml — per-env encrypted Helm values
- infra/kargo/values/*.enc.yaml — per-env encrypted Kargo admin secrets
- kargo/credentials/*.enc.yaml — per-env encrypted git credentials (ksops)
- infra/kargo-credentials/ — ArgoCD app for deploying Kargo creds via ksops
- All repoURLs point to deploy-app-kargo-private

Structure from deploy-app-kargo (reference), adapted for SOPS workflow

kargo/credentials/ksops-generator.prod.yaml

@@ -0,0 +1,14 @@
+# ksops generator for PROD cluster
+# Replace ksops-generator.yaml on infra/stage/prod branch
+apiVersion: viaduct.ai/v1
+kind: ksops
+metadata:
+  name: kargo-git-credentials
+  annotations:
+    config.kubernetes.io/function: |
+      exec:
+        path: ksops
+files:
+  - git-creds-infra.prod.enc.yaml
+  - git-creds-ci.prod.enc.yaml
+  - git-creds-test-env.prod.enc.yaml