Add category suggestions to the post editor.

specs/editor_post.allium

@@ -37,7 +37,7 @@ value PostEditorMetadata {
     language: String?                   -- select from supported languages
     do_not_translate: Boolean           -- checkbox
     slug: String                        -- read-only text input
-    categories: List<String>            -- chip input
+    categories: List<String>            -- autocomplete chip input
     template_slug: String?              -- select (shown only when templates exist)
     post_links: PostLinksPanel
     linked_media: List<LinkedMediaItem>
@@ -149,6 +149,13 @@ surface PostEditorSurface {
         --   it is an index read (no model inference) so it works offline and
         --   yields nothing when similarity is disabled or unindexed.
 
+    @guarantee CategoryAutocomplete
+        -- Category input with autocomplete.
+        -- While typing: substring match on existing category names (case-insensitive),
+        --   top 8 shown, plus a "create category" row when the query is new.
+        -- Selecting or creating a category adds it to the post and makes newly
+        --   created categories available in project metadata for future suggestions.
+
     @guarantee TranslationFlagsBar
         -- Row of flag emoji buttons inline with metadata toggle.
         -- One flag per language: canonical language + each translation.