refactor: finish legacy room cleanup and explicit migration boundary

setup/migrate-room-registrations.test.ts

@@ -149,6 +149,16 @@ describe('migrate room registrations step', () => {
         agent_type: 'claude-code',
       },
     ]);
+    expect(
+      migratedDb
+        .prepare(
+          `SELECT name
+             FROM sqlite_master
+            WHERE type = 'table'
+              AND name = 'registered_groups'`,
+        )
+        .get(),
+    ).toBeUndefined();
     expect(
       migratedDb
         .prepare(