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/common.go

@@ -606,37 +606,12 @@ func GetRunningImageTag(appName string, imageTag string) (string, error) {
 
 // GetDokkuAppShell returns the shell for a given app
 func GetDokkuAppShell(appName string) string {
-	shell := "/bin/bash"
-	globalShell := ""
-	appShell := ""
-
-	ctx := context.Background()
-	errs, ctx := errgroup.WithContext(ctx)
-	errs.Go(func() error {
-		results, _ := CallPlugnTriggerWithContext(ctx, PlugnTriggerInput{
-			Trigger: "config-get-global",
-			Args:    []string{"DOKKU_APP_SHELL"},
-		})
-		globalShell = results.StdoutContents()
-		return nil
-	})
-	errs.Go(func() error {
-		results, _ := CallPlugnTriggerWithContext(ctx, PlugnTriggerInput{
-			Trigger: "config-get",
-			Args:    []string{appName, "DOKKU_APP_SHELL"},
-		})
-		appShell = results.StdoutContents()
-		return nil
-	})
-
-	errs.Wait()
+	appShell := PropertyGet("scheduler", appName, "shell")
 	if appShell != "" {
-		shell = appShell
-	} else if globalShell != "" {
-		shell = globalShell
+		return appShell
 	}
 
-	return shell
+	return PropertyGetDefault("scheduler", "--global", "shell", "/bin/bash")
 }
 
 // DokkuApps returns a list of all local apps