fix: fixed tag suggestions

specs/editor_post.allium

@@ -137,10 +137,14 @@ surface PostEditorSurface {
 
     @guarantee TagAutocomplete
         -- Tag input with autocomplete.
-        -- Standard: prefix match on existing tag names (case-insensitive).
-        -- When semanticSimilarityEnabled: also suggests tags from 10 similar posts,
-        --   weighted by similarity, top 5 shown.
-        -- Merged results: prefix matches first, then embedding suggestions.
+        -- While typing: substring match on existing tag names (case-insensitive),
+        --   top 8 shown, plus a "create tag" row when the query is new.
+        -- While the query is empty (on focus): semantic suggestions appear under a
+        --   "Suggested tags" label — tags drawn from up to 10 similar posts,
+        --   weighted by similarity, top 5, excluding tags already on the post.
+        --   Requires semanticSimilarityEnabled and a built embedding index;
+        --   it is an index read (no model inference) so it works offline and
+        --   yields nothing when similarity is disabled or unindexed.
 
     @guarantee TranslationFlagsBar
         -- Row of flag emoji buttons inline with metadata toggle.