docs(updating): document automated migration tool and zone.js polyfills setup for v9

docs/updating/9-0.md

@@ -12,6 +12,32 @@ This guide assumes that you have already updated your app to the latest version
 For a **complete list of breaking changes** from Ionic 8 to Ionic 9, please refer to [the breaking changes document](https://github.com/ionic-team/ionic-framework/blob/main/BREAKING.md#version-9x) in the Ionic Framework repository.
 :::
 
+## Automated Migration
+
+Before manually working through the changes below, you can run the Ionic migration tool. It scans your app, automatically applies the breaking changes that can be safely migrated, and prints a checklist of the remaining updates that require manual work. Each item includes the affected file, line number, and a link to the corresponding section of this guide. Because the tool reads your framework and version from `package.json`, it only applies migrations that are relevant to your app.
+
+:::warning
+Commit your work first. The tool writes changes in place and refuses to run on a dirty working tree, or a project without git, unless you pass `--force`, so git is your undo.
+:::
+
+```shell
+npx @ionic/migrate
+```
+
+After a run it prints a summary of what it fixed and what is left for you, formats the changed files with your project's Prettier, and, if it applied anything, reinstalls dependencies so `node_modules` matches the version bump.
+
+Useful options:
+
+- `--dry-run` reports what would change without writing anything.
+- `--check` reports only and exits non-zero if any migration is still pending, for use in CI.
+- `--force` writes even if the working tree is dirty or the project is not a git repository.
+- `--no-format` skips the Prettier pass over the changed files.
+- `--no-install` skips the dependency reinstall.
+
+Run `npx @ionic/migrate --help` for the full list.
+
+The tool is single-shot. Once it bumps your `@ionic/*` version, a re-run detects the new major and does nothing, so run it once per major upgrade and review the diff before committing.
+
 ## Getting Started
 
 ### Angular
@@ -72,12 +98,20 @@ platformBrowserDynamic()
   .catch((err) => console.error(err));
 ```
 
-Either way, also confirm `zone.js` is listed in the `polyfills` array in `angular.json`. Angular 21 and later default scaffolds omit it:
+Either way, also confirm `zone.js` is still loaded, since Angular 21 and later default scaffolds omit it. Add it back through whichever polyfills setup your project uses.
+
+If `angular.json` lists polyfills as an array, include `zone.js`:
 
 ```json title="angular.json"
 "polyfills": ["zone.js"]
 ```
 
+If your project uses a polyfills file instead (for example, Ionic starters set `"polyfills": "src/polyfills.ts"`), import it there:
+
+```ts title="src/polyfills.ts"
+import 'zone.js';
+```
+
 #### OnPush Change Detection on Angular 22
 
 Angular 22 changes the default change detection strategy to `OnPush` for components that don't declare one. Combined with the zoneless default above, component state you mutate as a plain field from an Ionic lifecycle hook (`ionViewWillEnter`, and so on) no longer re-renders on its own.