feat(agent): persist assistant model preference

drizzle/0025_assistant_model.sql

@@ -0,0 +1 @@
+ALTER TABLE "users" ADD COLUMN "assistant_model" text;

drizzle/meta/_journal.json

@@ -176,6 +176,13 @@
       "when": 1780394000000,
       "tag": "0024_user_assistant_customization",
       "breakpoints": true
+    },
+    {
+      "idx": 25,
+      "version": "7",
+      "when": 1783560000000,
+      "tag": "0025_assistant_model",
+      "breakpoints": true
     }
   ]
 }