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/src/prop/prop.go

@@ -192,6 +192,76 @@ func main() {
 			fmt.Fprintln(os.Stderr, err.Error())
 			os.Exit(1)
 		}
+	case "migrate-config-to-property":
+		property := flag.Arg(2)
+		configVar := flag.Arg(3)
+
+		globalConfigVar := ""
+		transformName := ""
+		listProperty := false
+		for i := 4; i < flag.NArg(); i++ {
+			arg := flag.Arg(i)
+			switch {
+			case arg == "--list":
+				listProperty = true
+			case arg == "--global-config-var" && i+1 < flag.NArg():
+				i++
+				globalConfigVar = flag.Arg(i)
+			case arg == "--transform" && i+1 < flag.NArg():
+				i++
+				transformName = flag.Arg(i)
+			}
+		}
+
+		var transform func(string) string
+		switch transformName {
+		case "restore-bool":
+			transform = func(value string) string {
+				if value == "0" {
+					return "false"
+				}
+				return "true"
+			}
+		case "non-empty-to-true":
+			transform = func(value string) string {
+				if value != "" {
+					return "true"
+				}
+				return value
+			}
+		case "skip-all-checks-to-disabled":
+			transform = func(value string) string {
+				if value == "true" {
+					return "_all_"
+				}
+				return ""
+			}
+		case "checks-enabled-to-skipped":
+			transform = func(value string) string {
+				if value == "0" {
+					return "_all_"
+				}
+				return ""
+			}
+		case "":
+			// no transform
+		default:
+			fmt.Fprintf(os.Stderr, "Unknown transform: %s\n", transformName)
+			os.Exit(1)
+		}
+
+		entry := common.MigrateConfigEntry{
+			ConfigVar:       configVar,
+			GlobalConfigVar: globalConfigVar,
+			Property:        property,
+			Transform:       transform,
+			ListProperty:    listProperty,
+		}
+
+		if err := common.MigrateConfigToProperties(pluginName, []common.MigrateConfigEntry{entry}); err != nil {
+			fmt.Fprintln(os.Stderr, err.Error())
+			os.Exit(1)
+		}
 	case "set":
 		appName := flag.Arg(2)
 		property := flag.Arg(3)