Add section on Zod 4 to v6 migration guide

src/content/docs/en/guides/upgrade-to/v6.mdx

@@ -111,6 +111,40 @@ Integration and adapter maintainers should pay special attention to changes affe
 - [routes with percent-encoded percent signs (e.g. `%25`)](#removed-percent-encoding-in-routes)
 - [`astro:ssr-manifest` virtual module](#removed-astrossr-manifest-virtual-module-integration-api)
 
+### Zod 4
+
+Astro v6.0 upgrades to Zod 4, a major dependency update that may require changes to custom Zod schemas in your project.
+
+#### What should I do?
+
+If you have custom Zod schemas in your `content.config.ts` or other configuration files, you'll need to update them for Zod 4. Refer to the [Zod migration guide](https://zod.dev/v4/changelog) for detailed changes in the Zod API.
+
+Notably, many `string()` formats have been deprecated (e.g. `z.string().email()`, `z.string.url()`), and their APIs have been moved to the top-level z namespace. You may need to update how you validate form input for your Astro Actions:
+
+ ```ts title="src/actions/index.ts" ins={8} del={7}
+  import { defineAction } from 'astro:actions';
+  import { z } from 'astro/zod';
+  export const server = {
+    newsletter: defineAction({
+      accept: 'form',
+      input: z.object({
+        email: z.string().email(),
+        email: z.email(),
+        terms: z.boolean(),
+      }),
+      handler: async ({ email, terms }) => { /* ... */ },
+    })
+  }
+  ```
+
+You can ensure you're the same version of Zod that Astro uses internally by [importing Zod from `astro/zod`](#deprecated-astroschema-and-z-from-astrocontent).
+
+```ts
+import { z } from 'astro/zod';
+```
+
+<ReadMore>See more about [the `astro/zod` module](/en/reference/modules/astro-zod/).</ReadMore>
+
 ## Legacy
 
 The following features are now considered legacy features. They should function normally but are no longer recommended and are in maintenance mode. They will see no future improvements and documentation will not be updated. These features will eventually be deprecated, and then removed entirely.