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

infra/gitea-custom/config.yaml

@@ -0,0 +1,10 @@
+{
+  "name": "gitea-custom",
+  "namespace": "gitea",
+  "step": "6",
+  "source": {
+    "repoURL": "https://github.com/Kargones/deploy-app-kargo-private.git",
+    "path": "infra/gitea-custom/manifests",
+    "targetRevision": "main"
+  }
+}