Add engagement-only conversation mode

tests/database.test.ts

@@ -130,6 +130,10 @@ describe('Database Client', () => {
         'account_browsing_settings',
         'account_browsing_decisions',
       ]));
+      const learningTable = db.prepare(
+        "SELECT sql FROM sqlite_master WHERE type = 'table' AND name = 'conversation_learning_trajectories'",
+      ).get() as { sql: string };
+      expect(learningTable.sql).toContain("'engagement'");
     });
 
     it('tracks schema version and migrates a legacy sent_messages table transactionally', () => {