feat: Move cluster services to wild-directory as unified packages

Convert all 15 cluster services from embedded API format to
wild-directory packages using the unified manifest format:
- metallb, traefik, cert-manager, longhorn, snapshot-controller
- nfs, smtp, coredns, node-feature-discovery, nvidia-device-plugin
- externaldns, docker-registry, headlamp, crowdsec, utils

Changes:
- wild-manifest.yaml → manifest.yaml with is, defaultConfig, requires
- Eliminated configReferences and serviceConfig fields
- Flattened kustomize.template/ to package root
- Template vars use flat defaultConfig keys
- install.sh paths updated for apps/ layout
- Updated 9 app manifests: cloud.smtp.* → apps.smtp.* with requires
- Removed dead install: true field from 6 app manifests

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

keila/manifest.yaml

@@ -5,6 +5,7 @@ version: 0.17.1
 icon: https://cdn.jsdelivr.net/gh/homarr-labs/dashboard-icons/svg/keila.svg
 requires:
   - name: postgres
+  - name: smtp
 defaultConfig:
   namespace: keila
   externalDnsDomain: "{{ .cloud.domain }}"
@@ -20,12 +21,12 @@ defaultConfig:
   adminUser: admin@{{ .cloud.domain }}
   tlsSecretName: wildcard-wild-cloud-tls
   smtp:
-    host: "{{ .cloud.smtp.host }}"
-    port: "{{ .cloud.smtp.port }}"
-    from: "{{ .cloud.smtp.from }}"
-    user: "{{ .cloud.smtp.user }}"
-    tls: "{{ .cloud.smtp.tls }}"
-    startTls: "{{ .cloud.smtp.startTls }}"
+    host: "{{ .apps.smtp.host }}"
+    port: "{{ .apps.smtp.port }}"
+    from: "{{ .apps.smtp.from }}"
+    user: "{{ .apps.smtp.user }}"
+    tls: "{{ .apps.smtp.tls }}"
+    startTls: "{{ .apps.smtp.startTls }}"
 defaultSecrets:
   - key: secretKeyBase
     default: "{{ random.AlphaNum 64 }}"