feat: migrate environment variables to plugin properties

Standardize how environment variables are migrated to properties during
install triggers and migrate all remaining DOKKU_* config vars to their
appropriate plugin properties.

Adds a reusable MigrateConfigToProperties() function in the common
package with Transform callback and ListProperty support, plus a
migrate-config-to-property subcommand for the prop binary so shell
plugins can use the same code path.

Migrated variables and their new property owners:
- DOKKU_APP_PROXY_TYPE/DOKKU_PROXY_TYPE → proxy type
- DOKKU_DISABLE_PROXY → proxy disabled
- DOKKU_PROXY_PORT → ports proxy-port
- DOKKU_PROXY_SSL_PORT → ports proxy-ssl-port
- DOKKU_APP_RESTORE → ps restore
- DOKKU_SKIP_DEPLOY → ps skip-deploy
- DOKKU_START_CMD → ps start-cmd
- DOKKU_DOCKERFILE_START_CMD → ps dockerfile-start-cmd
- DOKKU_DISABLE_APP_AUTOCREATION → apps disable-autocreation
- DOKKU_APP_SHELL → scheduler shell
- DOKKU_SKIP_CLEANUP → builder skip-cleanup
- DOKKU_CHECKS_DISABLED → checks disabled
- DOKKU_CHECKS_SKIPPED → checks skipped
- DOKKU_CHECKS_WAIT → checks wait
- DOKKU_CHECKS_TIMEOUT → checks timeout
- DOKKU_CHECKS_ATTEMPTS → checks attempts
- DOKKU_DEFAULT_CHECKS_WAIT → checks default-wait
- DOKKU_SKIP_ALL_CHECKS → checks disabled (legacy)
- DOKKU_SKIP_DEFAULT_CHECKS → checks skipped (legacy)

Also refactors existing bespoke migration loops in scheduler, ports, ps,
builder, and nginx-vhosts plugins to use the standardized utility.

Removes DOKKU_PARALLEL_ARGUMENTS from documentation (unused in core).
Deprecates fn-migrate-config-to-property bash function.

closes #1558

plugins/config/config.go

@@ -62,7 +62,7 @@ func SetMany(appName string, entries map[string]string, replace bool, restart bo
 		})
 		triggerUpdate(appName, "set", keys)
 	}
-	if !global && restart && env.GetBoolDefault("DOKKU_APP_RESTORE", true) {
+	if !global && restart && shouldRestart(appName) {
 		triggerRestart(appName)
 	}
 	return
@@ -98,7 +98,7 @@ func UnsetMany(appName string, keys []string, restart bool) (err error) {
 		})
 		triggerUpdate(appName, "unset", keys)
 	}
-	if !global && restart && env.GetBoolDefault("DOKKU_APP_RESTORE", true) {
+	if !global && restart && shouldRestart(appName) {
 		triggerRestart(appName)
 	}
 	return
@@ -125,12 +125,20 @@ func UnsetAll(appName string, restart bool) (err error) {
 		})
 		triggerUpdate(appName, "clear", []string{})
 	}
-	if !global && restart && env.GetBoolDefault("DOKKU_APP_RESTORE", true) {
+	if !global && restart && shouldRestart(appName) {
 		triggerRestart(appName)
 	}
 	return
 }
 
+func shouldRestart(appName string) bool {
+	results, _ := common.CallPlugnTrigger(common.PlugnTriggerInput{
+		Trigger: "ps-get-property",
+		Args:    []string{appName, "restore"},
+	})
+	return results.StdoutContents() != "false"
+}
+
 func triggerRestart(appName string) {
 	if !common.IsDeployed(appName) {
 		return