refactor(stacks): 🚚 migrate sizer-receiver from garm to ci-sizer namespace

Move sizer-receiver ArgoCD app and manifests from stacks/garm/ to
stacks/ci-sizer/. The sizer is provider-agnostic and no longer
belongs in the GARM-specific stack.

- destination namespace: garm → ci-sizer
- ArgoCD source path: stacks/garm/ → stacks/ci-sizer/
- ingress namespace: garm → ci-sizer
- GARM_URL unchanged (garm.garm.svc.cluster.local) — GARM server stays in its namespace
- Secrets (sizer-tokens, sizer-oidc-client, garm-fixed-credentials) must exist in ci-sizer namespace

template/stacks/ci-sizer/sizer-receiver.yaml

@@ -18,8 +18,8 @@ spec:
       limit: -1
   destination:
     name: in-cluster
-    namespace: garm
+    namespace: ci-sizer
   source:
     repoURL: https://{{{ .Env.CLIENT_REPO_DOMAIN }}}/{{{ .Env.CLIENT_REPO_ORG_NAME }}}
     targetRevision: HEAD
-    path: "{{{ .Env.CLIENT_REPO_ID }}}/{{{ .Env.DOMAIN }}}/stacks/garm/sizer-receiver"
+    path: "{{{ .Env.CLIENT_REPO_ID }}}/{{{ .Env.DOMAIN }}}/stacks/ci-sizer/sizer-receiver"

template/stacks/ci-sizer/sizer-receiver/ingress.yaml

@@ -10,7 +10,7 @@ metadata:
     dns.gardener.cloud/ttl: "600"
 {{{ end }}}
   name: sizer-receiver
-  namespace: garm
+  namespace: ci-sizer
 spec:
   ingressClassName: nginx
   rules: