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/common/docker.go

@@ -390,23 +390,17 @@ func DockerBin() string {
 func DockerCleanup(appName string, forceCleanup bool) error {
 	if !forceCleanup {
 		skipCleanup := false
-		if appName != "" {
-			triggerName := "config-get"
-			triggerArgs := []string{appName, "DOKKU_SKIP_CLEANUP"}
-			if appName == "--global" {
-				triggerName = "config-get-global"
-				triggerArgs = []string{"DOKKU_SKIP_CLEANUP"}
-			}
-
-			results, _ := CallPlugnTrigger(PlugnTriggerInput{
-				Trigger: triggerName,
-				Args:    triggerArgs,
-			})
-			if results.StdoutContents() == "true" {
+		if appName != "" && appName != "--global" {
+			if PropertyGet("builder", appName, "skip-cleanup") == "true" {
 				skipCleanup = true
 			}
 		}
 
+		if !skipCleanup && PropertyGet("builder", "--global", "skip-cleanup") == "true" {
+			skipCleanup = true
+		}
+
+		// Fallback for .dokkurc compatibility
 		if skipCleanup || os.Getenv("DOKKU_SKIP_CLEANUP") == "true" {
 			LogInfo1("DOKKU_SKIP_CLEANUP set. Skipping dokku cleanup")
 			return nil