cloud-init final_message: no $hostname (not expanded there; warned on every deploy)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/rollcall/deploy.go

@@ -286,7 +286,7 @@ func (a *App) renderSeed(j *Job, sp DeploySpec, token string) []seedFile {
 			fmt.Fprintf(&b, "packages: %s\n", yamlList(sp.Packages))
 		}
 		fmt.Fprintf(&b, "phone_home:\n  url: %s/api/v1/phone-home/%s\n  post: [instance_id, hostname, fqdn, pub_key_ed25519]\n  tries: 10\n", base, token)
-		b.WriteString("final_message: \"rollcall: deployed $hostname up after $UPTIME s\"\n")
+		b.WriteString("final_message: \"rollcall: deployed, up after $UPTIME s\"\n") // only $UPTIME/$TIMESTAMP/$DATASOURCE/$VERSION are expanded
 		ud = b.String()
 	}
 	var frags []TagDef