docs: Update ADDING-APPS.md to remove cloud.smtp references

SMTP config is now at apps.smtp.* via the SMTP infrastructure app,
not cloud.smtp.*. Remove the old variable listing and update the
configuration flow documentation.

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

ADDING-APPS.md

@@ -121,15 +121,6 @@ Here's a comprehensive rundown of all config variables that get set during clust
 
 - cloud.dockerRegistryHost - Docker registry hostname (e.g., "registry.internal.cloud2.payne.io")
 
-##### SMTP Configuration (SMTP Service):
-
-- cloud.smtp.host - SMTP server hostname
-- cloud.smtp.port - SMTP port (typically "465" or "587")
-- cloud.smtp.user - SMTP username
-- cloud.smtp.from - Default 'from' email address
-- cloud.smtp.tls - Enable TLS (true/false)
-- cloud.smtp.startTls - Enable STARTTLS (true/false)
-
 ###### Backup Configuration:
 
 - cloud.backup.root - Root path for backups
@@ -214,8 +205,7 @@ Configuration Flow
    - ExternalDNS → cluster.externalDns.ownerId
    - NFS → cloud.nfs.*
    - Docker Registry → cloud.dockerRegistryHost, cluster.dockerRegistry.storage
-   - SMTP → cloud.smtp.*
-4. Apps: Each app adds its configuration under apps.<name>.* based on its manifest
+4. Apps: Each app adds its configuration under apps.<name>.* based on its manifest (including SMTP as an infrastructure app at apps.smtp.*)
 
 #### Manifest App Reference Resolution: