test(ralph): pin the case name a Ralph loop start registers with

Merge-time fix for #676 (review finding 2). POST /api/ralph-loop/start now
passes { caseName } to addSession so the session lands in its case's default
session group, and nothing tested it. The registration-order test asserts the
placement argument; it fails against the route without the change.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

test/routes/ralph-routes.test.ts

@@ -476,6 +476,8 @@ describe('ralph-routes', () => {
         payload: { taskDescription: 'test task', completionPhrase: 'DONE', caseName: 'registration-order' },
       });
       await vi.waitFor(() => expect(harness.ctx.addSession).toHaveBeenCalledTimes(1));
+      // The case rides the registration, so the session lands in the case's default session group.
+      expect(harness.ctx.addSession).toHaveBeenCalledWith(expect.anything(), { caseName: 'registration-order' });
       expect(harness.ctx.persistSessionState).not.toHaveBeenCalled();
       expect(harness.ctx.setupSessionListeners).not.toHaveBeenCalled();