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/git-creds-test-env.dev.enc.yaml

@@ -0,0 +1,36 @@
+apiVersion: v1
+kind: Secret
+metadata:
+    name: github-creds
+    namespace: test-env
+    labels:
+        kargo.akuity.io/cred-type: git
+type: Opaque
+stringData:
+    repoURL: https://github.com/Kargones/deploy-app-kargo-private.git
+    username: Kargones
+    password: ENC[AES256_GCM,data:pGwKoR6eurha3ATmL0XP01DTK3qXCxAV3SdFYmNFokKMZI+VuzZ/GQ==,iv:DKVdeBRQS+HjPkHftJZZuXf7yB+I18Ae3iK+7L+hZQ0=,tag:/K6twgUKTNrn6s1rDTAeQA==,type:str]
+sops:
+    age:
+        - recipient: age1xmnaqlrjzpk5hl7uhel9sehqh7zdz8p59qte2myt97aqd7lyeuxszuess7
+          enc: |
+            -----BEGIN AGE ENCRYPTED FILE-----
+            YWdlLWVuY3J5cHRpb24ub3JnL3YxCi0+IFgyNTUxOSBYTk5seDNiR2dRcXRRclJZ
+            UlNuRXZGd3ZMTDNsaHVrYUtnUmsyNk1IVkJFCmo0Y0pwdnlsay9qcy8rakk4ejBF
+            VHAzbmRObUFVVVpZN0JraHRlVmhhNW8KLS0tIHNXd01OUnVxclRXcE9iNFNzZS9S
+            UlQzRnlSb2F0Y0J0eVdZN1BSelRrSWcK8V99m3Hh2mOm9csGG42k4FIeFbQnkXIr
+            w3elSENAib0rFz7Uewpn1VD3yjFL7SVv33bVOEuM3KlHDtUnF6fuuQ==
+            -----END AGE ENCRYPTED FILE-----
+        - recipient: age1ame2tp44sq9rmkqzqvxy77eu7qd2035kmlgcsfjfxj2jughv3clqlku03g
+          enc: |
+            -----BEGIN AGE ENCRYPTED FILE-----
+            YWdlLWVuY3J5cHRpb24ub3JnL3YxCi0+IFgyNTUxOSBOS05ZanNzMWRFRG5EUlNq
+            WmtLUHpHRE0rWXRuVXRLRDFiRDZSZmdEc2lnCmMrbHRxRUhIWDFkaTJlekxrckxj
+            V3ROU25aOVZGVzVUY3IwRlhwYWhBWEUKLS0tIHZCS1ZobFlqQ2NZSUt3N1VSV2l4
+            SmVWWDRhckRxek1jRFpPUnIxb0QxaDQKmkVXDqP0tjaWo/fkdqrIatV/bhyQt9vb
+            Rzz21KATjKow6VtekA3FVOVchlDsaYDLkhuLI69I1lIYqR99ovS+8w==
+            -----END AGE ENCRYPTED FILE-----
+    lastmodified: "2026-03-11T06:58:02Z"
+    mac: ENC[AES256_GCM,data:CDESJXzhctA4WexUb2hP6ykWkRCWUsgIYHOSmRnHUlyf6Pd8uF43Bl/+ZHWhE6SLWf3LKGMOKSU60Qkh1aSH+Z9BqaiKa3dL0MIQx4yXRe9NzS/mEki4IsuQi27AZ6BKD93tznyVKbCAzxmyP1jRhbRNIKBA2dWoQAITX1w2vvA=,iv:l8JZK3a17DfIn3llapKnUZH7SRNCte5MNPZzjtPJbxA=,tag:rEcarIr7UhsV+THixTUiKw==,type:str]
+    encrypted_regex: ^(password|token|secret|key|privateKey|admin-password|db-password|passwordHash|tokenSigningKey)$
+    version: 3.12.1