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-ci.dev.enc.yaml

@@ -0,0 +1,36 @@
+apiVersion: v1
+kind: Secret
+metadata:
+    name: github-creds
+    namespace: ci
+    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:m9CZFhTHwaX6TrpGtbAynbI7eiprSrhCVVtST6fWiduFP55/EWtx1Q==,iv:iAjeVVakD8SZWhaEBu7JVR1YL2UGzYNBoy1W13M+Jwg=,tag:ABSswqyDhJG8+zlu8dWrgg==,type:str]
+sops:
+    age:
+        - recipient: age1xmnaqlrjzpk5hl7uhel9sehqh7zdz8p59qte2myt97aqd7lyeuxszuess7
+          enc: |
+            -----BEGIN AGE ENCRYPTED FILE-----
+            YWdlLWVuY3J5cHRpb24ub3JnL3YxCi0+IFgyNTUxOSBHOWI4eWZxOERjRHVhV1VI
+            MVE3WDErejZrNzBaRnVHK280dXI4WThxR2tvCmVzbE1HR1MzcEgybm8vUVVSOWJY
+            VlVvREl4NVRDRElHVHk5aWpnVkVhNW8KLS0tIG5FOXRwRy8xakFvaVlBTXF5OXJJ
+            aHFlbWh0Ym1NWEswbFM5N01JRGtGMUUKyjiN+bK0+6PLc/LG/dozgrAhB4PLcZmV
+            JVtxrSZWV6RQOuvc/HUw2yhO3dJldYqWauX+7ZOy2o1JACdPe9ixdQ==
+            -----END AGE ENCRYPTED FILE-----
+        - recipient: age1ame2tp44sq9rmkqzqvxy77eu7qd2035kmlgcsfjfxj2jughv3clqlku03g
+          enc: |
+            -----BEGIN AGE ENCRYPTED FILE-----
+            YWdlLWVuY3J5cHRpb24ub3JnL3YxCi0+IFgyNTUxOSBHWU9JLzdwbDI1SDJ3MnlE
+            YnlRQkdzWExpa1FGNjArSHFCamc2bWJnbjBBCnlkQlVEQVNDTXMvZ3NWRzhyT2lM
+            R0FpVUdleUVUcVVrTnRxQjVnSFo3S2cKLS0tIG1MZ0hYdGJpdTJ3WWU3MU1WeWFr
+            cklvSldLdW5ERDF1aW95dDlxOUtGbDAK7uoTP8idcEM8rQ7rRashsbzc9f4S7qI9
+            Bl87wLgMAFYfFO41cwSqhMY+gr7HRkDQq65CWo8HpQjMMlneC72jVQ==
+            -----END AGE ENCRYPTED FILE-----
+    lastmodified: "2026-03-11T06:58:02Z"
+    mac: ENC[AES256_GCM,data:OYJAnGhgB0sOC2KEgse7ZYMkrvrRNsMWqVQZM29VbhPbvp3s3GMmJPROPydsjvm6OE4EIi0C6ry2h9pGoRY5IApw6+tSXVmrlr3oQIjkGqimJVdJ0oBSwEYtZ6gNKpsn1I7ZthXCHBfZmmESkYiTJj8ogdXxNLIqydIuITengpI=,iv:IpBxWcDeGUD9ucIAVjY3Ojc0wd63nWDPDIcH8OtvJY8=,tag:yDZ8d9E9dufB/iGTk71t2Q==,type:str]
+    encrypted_regex: ^(password|token|secret|key|privateKey|admin-password|db-password|passwordHash|tokenSigningKey)$
+    version: 3.12.1