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/checks/internal-functions

@@ -72,7 +72,7 @@ cmd-checks-report-single() {
 fn-checks-disabled-list() {
   declare APP="$1"
 
-  local DOKKU_CHECKS_DISABLED=$(config_get "$APP" DOKKU_CHECKS_DISABLED)
+  local DOKKU_CHECKS_DISABLED=$(fn-plugin-property-get-default "checks" "$APP" "disabled" "")
   DOKKU_CHECKS_DISABLED="${DOKKU_CHECKS_DISABLED:-none}"
   echo "$DOKKU_CHECKS_DISABLED"
 }
@@ -80,7 +80,7 @@ fn-checks-disabled-list() {
 fn-checks-skipped-list() {
   declare APP="$1"
 
-  local DOKKU_CHECKS_SKIPPED=$(config_get "$APP" DOKKU_CHECKS_SKIPPED)
+  local DOKKU_CHECKS_SKIPPED=$(fn-plugin-property-get-default "checks" "$APP" "skipped" "")
   DOKKU_CHECKS_SKIPPED="${DOKKU_CHECKS_SKIPPED:-none}"
   echo "$DOKKU_CHECKS_SKIPPED"
 }