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/test-env/project.yaml

@@ -0,0 +1,14 @@
+apiVersion: kargo.akuity.io/v1alpha1
+kind: Project
+metadata:
+  name: test-env
+---
+apiVersion: kargo.akuity.io/v1alpha1
+kind: ProjectConfig
+metadata:
+  name: test-env
+  namespace: test-env
+spec:
+  promotionPolicies:
+    - stageSelector: { name: dev }
+      autoPromotionEnabled: true