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/infra/warehouse.yaml

@@ -0,0 +1,22 @@
+apiVersion: kargo.akuity.io/v1alpha1
+kind: Warehouse
+metadata:
+  name: infra-charts
+  namespace: infra
+spec:
+  subscriptions:
+    - chart:
+        repoURL: https://charts.jetstack.io
+        name: cert-manager
+        semverConstraint: ">=1.17.0"
+        discoveryLimit: 5
+    - chart:
+        repoURL: https://argoproj.github.io/argo-helm
+        name: argo-rollouts
+        semverConstraint: ">=2.39.0"
+        discoveryLimit: 5
+    - chart:
+        repoURL: https://dl.gitea.com/charts
+        name: gitea
+        semverConstraint: ">=10.6.0"
+        discoveryLimit: 5