Update migration rules to use defineMigration

- schema-deprecation-pattern.md: Replace raw client.patch with
  defineMigration, at, setIfMissing, unset from sanity/migrate
- migration-html-import.md: Add defineMigration wrapper example
  for reproducible HTML imports

skills/sanity-best-practices/rules/migration-html-import.md

@@ -104,4 +104,36 @@ async function uploadImage(client, imageUrl) {
 }
 ```
 
-Reference: [Content Migration Cheatsheet](https://www.sanity.io/docs/content-lake/content-migration-cheatsheet)
+### Using in a Migration
+
+Wrap this in `defineMigration` for reproducible imports:
+
+```typescript
+// migrations/import-wordpress-posts/index.ts
+import {defineMigration, createOrReplace} from 'sanity/migrate'
+import {htmlToBlocks} from '@portabletext/block-tools'
+
+export default defineMigration({
+  title: 'Import WordPress posts',
+  async *migrate(documents, context) {
+    const posts = await fetchWordPressPosts() // Your import source
+    
+    for (const post of posts) {
+      const blocks = htmlToBlocks(post.content, blockContentType, {
+        parseHtml: html => new JSDOM(html).window.document,
+      })
+      
+      yield createOrReplace({
+        _id: `post-${post.slug}`,
+        _type: 'post',
+        title: post.title,
+        body: blocks,
+      })
+    }
+  }
+})
+```
+
+Run with: `sanity migration run import-wordpress-posts --no-dry-run`
+
+Reference: [Schema and Content Migrations](https://www.sanity.io/docs/content-lake/schema-and-content-migrations)

skills/sanity-best-practices/rules/schema-deprecation-pattern.md

@@ -49,19 +49,36 @@ defineField({
 2. Deploy schema changes
 
 **Phase 2: Migrate**
-1. Update frontend to use new fields (with fallbacks)
-2. Run migration script to move data:
+1. Update frontend to use new fields (with fallbacks using `coalesce()`)
+2. Create a migration file in `migrations/` folder:
 
 ```typescript
-// Migration script example
-const documents = await client.fetch(`*[_type == "article" && defined(oldTitle)]`)
+// migrations/rename-oldTitle-to-newTitle/index.ts
+import {defineMigration, at, setIfMissing, unset} from 'sanity/migrate'
 
-for (const doc of documents) {
-  await client.patch(doc._id)
-    .set({ newTitle: doc.oldTitle })
-    .unset(['oldTitle'])
-    .commit()
-}
+export default defineMigration({
+  title: 'Rename oldTitle to newTitle',
+  documentTypes: ['article'],
+  filter: 'defined(oldTitle) && !defined(newTitle)',
+  migrate: {
+    document(doc) {
+      return [
+        at('newTitle', setIfMissing(doc.oldTitle)),
+        at('oldTitle', unset())
+      ]
+    }
+  }
+})
+```
+
+3. Run the migration:
+
+```bash
+# Dry run first (default)
+sanity migration run rename-oldTitle-to-newTitle
+
+# Execute when ready
+sanity migration run rename-oldTitle-to-newTitle --no-dry-run
 ```
 
 **Phase 3: Remove**