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.prod.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:OHHKJ82Bh3UtuWJwtIr4dNnMDxweZEs/bHQQ7buy+zQM99f1SC4zEA==,iv:kn2P/KO5QKYhNxWKcbphmEmeJayWz/xA3NbfSq3T8K4=,tag:EzwiJHrK36SqRgISnzYmmw==,type:str]
+sops:
+    age:
+        - recipient: age1xmnaqlrjzpk5hl7uhel9sehqh7zdz8p59qte2myt97aqd7lyeuxszuess7
+          enc: |
+            -----BEGIN AGE ENCRYPTED FILE-----
+            YWdlLWVuY3J5cHRpb24ub3JnL3YxCi0+IFgyNTUxOSBnREI0NnE1Z0Z6ZE1HWG4x
+            aHhZeDBIYmNJNjZJbVNJR2Myb3Zyb092cW1NCkt0T3Rwamx0S1dudE1rM1ZCTDZv
+            ZDlJTmh5YjdUT3k1YlZlM2FOdzgrc3MKLS0tIFhNeFdrNkRBelZRUldqenhBVFAx
+            dHF0cUFmNXRHS3RkeTVVQzhIVTBXdm8K6c/K14oe/bPkaFCJi/OpoLj3q8RE20Hn
+            /1yeNCfRNkAPyOYQae6XgPI2xL5H+PfhMzRWYa6ebI/Pefl3n4WeSA==
+            -----END AGE ENCRYPTED FILE-----
+        - recipient: age16p0gwk8vt90vy2gm8jjca8rcyd2drv5526e997ukdelnv5ek8unqm0smuk
+          enc: |
+            -----BEGIN AGE ENCRYPTED FILE-----
+            YWdlLWVuY3J5cHRpb24ub3JnL3YxCi0+IFgyNTUxOSBBdFhPTWRZUEllR01sQzBM
+            RTQvbzM5ZnoyaEtHS21hS1BCWjJFNkZDZml3ClBPZHgvU2lCTHRwL01JUFRpY3pI
+            RTcvNkNZNXkwYkRVQzBWNVRuSllPcTAKLS0tIDMwTlFkcXR1QmNBNDVwQXdPUzR0
+            UlBIbE83UXEzcmVNWXhyOWRXUit4REUKxJQwLz/w4BGMW6OggbaG0kDv0CWIUQi6
+            H1gWD4HKT1JM8rI9GrBJRijmQhJvfn+s3PcRnimVa8sFUXJwR8ffrA==
+            -----END AGE ENCRYPTED FILE-----
+    lastmodified: "2026-03-11T06:58:02Z"
+    mac: ENC[AES256_GCM,data:0sm3v9oBiyy+3/es2piC9O3505QyaiylQSy7/7IkOYHcLQi5T0R6pXV+LlTMrJgxbvQg5PjyFxtGDF/MS9VM7iimDwJzAwC+Jv9Z9senKG6DwenUl8B/hHtXoWHGIxKrzR+0nnh756u+ftDTR5u7PDdd03UIn+n6fhpZ3tRB1q8=,iv:T3KPpdXz+8Bq0TBlIPR3+tFWHEBsE515rBWWDyM0VGM=,tag:+aBXzSUykrdazF6lXGNwHQ==,type:str]
+    encrypted_regex: ^(password|token|secret|key|privateKey|admin-password|db-password|passwordHash|tokenSigningKey)$
+    version: 3.12.1